Welp. I replaced the CPU and CPU cooler, and tried again. Same result. POST screen that goes blank after a split-second, and four beeps.

As I was installing the new cooler, I noticed some sort of residue on small areas of the motherboard. Pictures attached.

I checked the pictures in the eBay listing, and sure enough, the residue was already there before the board was sold to me.

Anyone know what it is? Could it be (part of) the problem?

In addition, when I unpacked the new CPU, I realized that it did not come packed in an anti-static bag - just bubble wrap and a small foam pad, inside a hard plastic clamshell case. Also, the heat spreader had a number of scratches and fingerprints on it, as well as three blue Chinese characters stamped onto it (the latter were visible in the listing, which made me a little suspicious, but the seller had a high rating and claimed it tested good, so I took a chance, figuring I could take advantage of eBay's money back guarantee if it turned out to be bad).

On top of all that, when I tried to put the board in the case, I realized that the case fan obstructs the new CPU fan. Not sure what to do about that, short of getting a new case and/or a new cooler.

Unless someone has a better idea, I think my next step is going to be trying a different power supply. I've already ordered one.

That looks like dust to me.
I think your new CPU is probably fine since it's doing the same thing as the original one was. When you removed the heatsink, did the thermal paste look as though there was a tight fit between the heatsink and the CPU? If you know the heatsink if fitted correctly, it might be overvolted as suggested here: https://www.asus.com/support/faq/1029959/ Have you tried clearing the CMOS?

Did you put the Clear cmos jumper back right?

have you tried removing all of the ram sticks and trying them 1 by 1 in different ram slots? If not u should really do this.

CPU fan looks correct (for the old CPU & HSF combo that is). Maybe you could show us some pictures of how the thermal paste is spread and how your new cooler is mounted (both top and bottom)? Those white stuffs in your pictures are just dusts, you can wipe them off with cotton bud + alcohol. If the HSF is tightly and correctly mounted, and the fan is plugged in correctly, then it really doesn't sound good, as hardware issues could be developped on the board.

Repo Man11 wrote on 2020-08-29, 01:05:

That looks like dust to me.
I think your new CPU is probably fine since it's doing the same thing as the original one was. When you removed the heatsink, did the thermal paste look as though there was a tight fit between the heatsink and the CPU? If you know the heatsink if fitted correctly, it might be overvolted as suggested here: https://www.asus.com/support/faq/1029959/ Have you tried clearing the CMOS?

It looked like the old heatsink and CPU were making contact, but the paste definitely wasn't covering the entire heat spreader. When I installed the new one, I applied the paste in an X pattern, since I've read that you can get more even coverage that way. Didn't seem to solve the problem, though. And (while my experience with these things is limited) it does seem strange to me that the CPU would overheat to the point of shutdown that quickly, even if the cooler wasn't seated 100% perfectly.

I have tried clearing the CMOS, and I've tried replacing the battery (I put the old one back in when that didn't work).

If it's overvolted, what would be causing that problem and how can I solve it? Is that something that switching out the power supply might fix?

Warlord wrote on 2020-08-29, 01:12:

Did you put the Clear cmos jumper back right?

have you tried removing all of the ram sticks and trying them 1 by 1 in different ram slots? If not u should really do this.

I'm pretty sure I put the jumper back right, yeah, but I can double check next time I work on it. That might be a couple days from now - at this point, I think it might be prudent to wait until the new power supply comes before trying again.

I've been testing with a single RAM stick inserted, but always with the same stick in the same slot. I can try varying it in the future. You think a RAM problem could be causing this?

kalohimal wrote on 2020-08-29, 01:45:

CPU fan looks correct (for the old CPU & HSF combo that is). Maybe you could show us some pictures of how the thermal paste is spread and how your new cooler is mounted (both top and bottom)? Those white stuffs in your pictures are just dusts, you can wipe them off with cotton bud + alcohol. If the HSF is tightly and correctly mounted, and the fan is plugged in correctly, then it really doesn't sound good, as hardware issues could be developped on the board.
asus p5pe-vm.jpg

Too exhausted to open stuff up to take pictures of it right this moment, but I'll do so when the new PSU comes.

I did notice, as I was installing the new cooler, that its back plate comes kind of worryingly close to some solder points on the back of the motherboard. It didn't look like it was actually making direct contact with any of them, but idk, is it possible something could arc? Or that the board could flex enough to close the gap?

Also, it was tricky getting all of the screws to line up properly, and I had to nudge a couple of them to get them to go into their holes. It's possible that something got cross-threaded and prevented the heatsink from getting as tight to the CPU as it's supposed to be.

As I said above, I applied the new thermal paste in an X pattern. It's pretty thick, though, and I was a bit worried I might be using too much, but none of it squeezed out the sides or anything.

Usually if an HSF comes with a metal back plate, it will have a piece of plastic sheet attached to prevent accidental shorting out the components. Once it is attached, you can wiggle and pull the HSF a bit to see if it is tight enough or loose. This might be asking the obvious, but did you inspect the bottom of the HSF (the part that contacts with the CPU) to make sure there is no plastic sheet or whatever? Some of them come with one that needs to be peeled off. Other than that, I really couldn't think of anything else short of the fan feedback circuit and/or the thermal sensor being damaged. Modern Intel and AMD CPUs have an in-built analog thermal diode that is usually connected to the super I/O chip, plus a digital thermal sensor that the BIOS could set the limit and monitor the core temperatures. Malfunction of the digital sensor is quite unlikely since everything is inside the CPU, but if the circuit of the analog sensor to the super I/O is damaged then you'd have this symptom. The fan speed is also monitored by the super I/O chip.

kalohimal wrote on 2020-08-29, 06:24:

Usually if an HSF comes with a metal back plate, it will have a piece of plastic sheet attached to prevent accidental shorting out the components. Once it is attached, you can wiggle and pull the HSF a bit to see if it is tight enough or loose. This might be asking the obvious, but did you inspect the bottom of the HSF (the part that contacts with the CPU) to make sure there is no plastic sheet or whatever? Some of them come with one that needs to be peeled off. Other than that, I really couldn't think of anything else short of the fan feedback circuit and/or the thermal sensor being damaged. Modern Intel and AMD CPUs have an in-built analog thermal diode that is usually connected to the super I/O chip, plus a digital thermal sensor that the BIOS could set the limit and monitor the core temperatures. Malfunction of the digital sensor is quite unlikely since everything is inside the CPU, but if the circuit of the analog sensor to the super I/O is damaged then you'd have this symptom. The fan speed is also monitored by the super I/O chip.

I definitely took the plastic sheet off of the part of the heat sink that contacts the CPU.

The new power supply came yesterday, so I'm going to try it out today. I was also thinking that maybe that caked-on dust could be shorting something out? So I'm going to try cleaning it off as well.

If neither of those things works, I'm going to assume there's a problem with the motherboard itself, and I'll see about getting another one.