If I'm reading your steps right, it sounds like the problem doesn't occur with the very same components outside of the case, but does occur inside. If so, then I'd check how you're attaching the motherboard to the case, since you could be getting unintentional electrical transmission. The motherboard should be separated from the case by something non-conductive, ideally the special little risers that come with cases. Those little paper washers that you use with the screws might help, too.

I thought that, too. Thing is, the motherboard was sitting on I guess four metal risers, and each screw did went through a red paper insulation ring (again, not sure what it's called, but I think we're talking about the same thing). I made sure plastic standoffs were used everywhere else. There is one corner, though, that doesn't have a mounting hole (because it's covered by a VRM module on the other side), but I don't believe the motherboard touches the case anywhere directly.

I've already moved the motherboard to another case (and it works like a charm there), but I'll try to make some photos of the case tonight.

Maybe the metal of the motherboard is touching the metal of the case elsewhere. I'd try removing all of the add-in cards except for the video card, as well as all peripherals (you don't need a keyboard or mouse just to check startup).

Odd if that's happening and the board is secured on risers. Check that your add on cards are also not touching bare metal anywhere as well.

I did have one case a few years ago that was causing havoc on a more modern build, it turned out one of the leads for one of the front panel buttons was slightly stripped and shorting against the frame of the case. You never know, maybe try disconnecting any front panel switches you don't need to test the machine and carefully go over the case wiring.

Osprey wrote:

Maybe the metal of the motherboard is touching the metal of the case elsewhere. I'd try removing all of the add-in cards except for the video card, as well as all peripherals (you don't need a keyboard or mouse just to check startup).

You reckon it might be possible the screws that are used to mount the motherboard to the case, somehow touch something on the motherboard itself that is causing some sort of short? I've seen motherboards having been tightened to cases too much and the place where the underside of the head of the screw (dunno what it's called but I'll just call it that, preferably without having to resort to acronyms that may make our more obscene members have associative thoughts about the kind of pron that has nothing to do with PCs 🤣) damage the PCB of the board. Couldn't that perhaps somehow damage some traces or something?

I've always wondered, so might as well ask now 😁

Osprey wrote:

Maybe the metal of the motherboard is touching the metal of the case elsewhere. I'd try removing all of the add-in cards except for the video card, as well as all peripherals (you don't need a keyboard or mouse just to check startup).

In all the steps I've described, the system only had a motherboard, a CPU, RAM, a videocard and a keyboard.

BeginnerGuy wrote:

I did have one case a few years ago that was causing havoc on a more modern build, it turned out one of the leads for one of the front panel buttons was slightly stripped and shorting against the frame of the case. You never know, maybe try disconnecting any front panel switches you don't need to test the machine and carefully go over the case wiring.

The only wire connected to the board was the PC speaker.

Tetrium wrote:

You reckon it might be possible the screws that are used to mount the motherboard to the case, somehow touch something on the motherboard itself that is causing some sort of short? I've seen motherboards having been tightened to cases too much and the place where the underside of the head of the screw (dunno what it's called but I'll just call it that, preferably without having to resort to acronyms that may make our more obscene members have associative thoughts about the kind of pron that has nothing to do with PCs 🤣) damage the PCB of the board. Couldn't that perhaps somehow damage some traces or something?

Now that you mention it, I think that I've done that once or twice... tightened the motherboard too closely to the case. I don't remember exactly what the result was, but I have definitely formed a conscious habit to tighten them only until I feel a little resistance.

These kinds of issues are probably less common nowadays, since cases are aluminum. My experiences with them seem to all come from the days when cases were steel (I particularly cherished my all-steel full tower, but, man, was it heavy; I took it to a LAN party once and never again). Of course, jheronimus' case is most likely steel, too, so it's not surprising if that's the problem.