A few thoughts.

0) Does your motherboard support an Intel DX4?

1) Are you using the DX4 with write-back or write-through L1 cache? Does your motherboard support L1 in WB mode?

2) Did you try to disable Parity Check in CMOS? I have found that some 486 MB's do not workw ell with parity check.

3) Try using non-parity RAM?

4) Even at 4 V, an Intel DX4 will overheat.

I replaced two caps that measured a pretty high ESR, and I also resoldered all the CPU/cache pins and some other ones that weren't looking that good. It changed nothing, as I expected.

feipoa:

0) The BIOS identifies the CPU as 486DX4(P24C), so I assume it does. On the other hand, it would support the DX4 Overdrive chip anyways, which is meant for DX2 mobos (that DX4 chip doesn't even have WB cache).
1) I've tested both, and it makes no difference whatsoever. I don't think my mobo supports WB cache, though. I've also tested an AMD DX4-90 chip (with my voltage adapter kit), same problem.
2) There's no option for disabling Parity Check in this BIOS.
3) I'm pretty sure that neither of the RAM sticks I tested are parity RAMs. Sometimes, depending on what program I run, I get "OFF BOARD PARITY ERROR" instead of simply "PARITY ERROR ????", which according to google is something not mobo-RAM related. Never getting these messages with a DX2...
4) Sure, but I can actually use the computer a bit with the specific DX4 one I have, at 5v. I do need heatsink+fan though, and I'm just lucky that it actually runs that "good" (it crashes if I run Duke3D for 5+ minutes).

The only thing left to test is another graphics card, and I have none. I'd rather get a new 486 mobo that I know supports DX4, which also has PCI slots.

Perhaps the reason the DX2 works and not the DX4 overdrive is related to the L1 cache size. DX2 has 8 KB, DX4 has 16 KB. AMD DX4-90 has 8 KB, so I would guess it should work. If you have confirmed that your DX4 chips work fine in other MB's, then I have no other ideas.

My DX2 66 came with heatsink when new

Where as my mates DX4 100 came with heatsink and fan.
Doubt that's the answer to errors but I wouldn't run the CPU without cooling for very long.

8bitbubsy wrote on 2016-06-15, 13:20:

I have a mysterious problem with my UC4913 ver2.0 486 motherboard when using a DX4 (it's fine with DX2)...

Maybe there is an opportunity to take a bios dump?
I only found 1.0, although I know there is 1.4, but it is not read correctly and does not work (on win3x.org)

One other thought. Could you loosen up the ram timings? Does your BIOS support that? For both cache and dram. Maybe the dx4 is stressing out the ram just a tiny bit more than desirable. For testing maybe adding a wait state on all of them

I Definately wouldn't run the regular dx4 on 5v for more than 1 minute. It just burns it internally. Maybe you could just pop in your DX2 again and try to run it at 40x2. Some of them actually run fine with those settings. And due to the higher bus speed, it's not all that slower than your dx4.

For example: I've got a board that runs a DX50 or dx2-66. Any dx2-80 will make things unstable. So i have to choose between the DX50 and dx2-66. They are about the same speed in practice, but the dx2-66 is more kind to the rest of the (VLB) system. The more boring choice, but it runs best.

led178 wrote on 2021-11-17, 09:48:

8bitbubsy wrote on 2016-06-15, 13:20:

I have a mysterious problem with my UC4913 ver2.0 486 motherboard when using a DX4 (it's fine with DX2)...

Maybe there is an opportunity to take a bios dump?
I only found 1.0, although I know there is 1.4, but it is not read correctly and does not work (on win3x.org)

I don't have this motherboard anymore, and it unfortunately seems like I never dumped its BIOS.

led178 wrote on 2021-11-17, 09:48:

8bitbubsy wrote on 2016-06-15, 13:20:

I have a mysterious problem with my UC4913 ver2.0 486 motherboard when using a DX4 (it's fine with DX2)...

Maybe there is an opportunity to take a bios dump?
I only found 1.0, although I know there is 1.4, but it is not read correctly and does not work (on win3x.org)

I have this motherboard.
Here are the bios and a photo of the chip eprom (27C512).
If you need some boot screens, i can upload...
SDumas

SDumas wrote on 2021-11-17, 16:45:

Here are the bios and a photo of the chip eprom (27C512).

Thanks a lot! It V1.6 from 06-08-94 !!!

SDumas wrote on 2021-11-17, 16:45:

Here are the bios and a photo of the chip eprom (27C512).

He's very funny, he doesn't even have the option to choose the order of boot devices.

I have already studied the topic about AMIBCP and there are no solutions (except for the external setup) yet.
The oldest 6.21 does not work with it anymore.
Thanks again!