I had a similar issue with a VLB 5429. In my case it was the VGA ROM was having issues. I pulled the chip and tested it outside the card by reading it repeatedly and everything appeared fine. But in the card it had issues. Replaced it with a new EEPROM with the same ROM flashed to it and it worked great.

In my case, I was able to isolate this by determining that if the system booted okay and shadowed the VGA ROM at startup correctly, the card would work reliably for long periods of time. But if I turned the shadow off, it would occasionally glitch, freeze the system, etc. Even with the shadowing it would still occasionally fail to boot, presumably because it got a bad copy of the ROM at startup.

Yours doesn't sound like the exact same issue as I had, but maybe similar?

One more thing...

While playing around with the card some more, I've noticed something weird. If I touch my finger to one of the empty memory expansion sockets - location circled in the picture - I can make the display go from the mostly-corrupted state as shown in the first picture in my earlier post, to the not-so-corrupted state shown in the second photo. Not pressing, simply by just placing my finger there.

Guld wrote on Yesterday, 16:43:

In my case it was the VGA ROM was having issues.

I'm hoping it's not the ROM, because I don't have any means of verifying whether it's good - I don't have a programmer (e.g. TL866, T48, etc.). Unless it can be rigged up to an Arduino or something. But I don't even know what kind of ROM it is; there don't appear to be any markings on the package! As you can see from the photo, the sticker only just covers the window area, so I doubt there's anything hiding under there. All I know is it's a 28-pin UV-erasable EPROM in a ceramic package.

Symptoms are consistent with a cold solder joint on a memory chip. Might be worth a closer inspection for damage.

Despite the total lack of markings on the chip - I took the chip out of the socket again, and even on the bottom there is nothing except a hand-written "5429" in what appears to be pencil - after some investigation I think the ROM is a 27256 EPROM.

I know it's 32KB, because that's the size of the ROM image file available to download on the The Retro Web, so it's a 256Kbit ROM. I can see that what should be the data pins on one of those (D0-D7) are all connected to the inputs of an adjacent 74LS244, which in turn has its outputs connected to the Data pins of the ISA edge connector. +5V and ground are also connected to what should be VCC/VPP and VSS. What should be OE and CE appear to have approx. 3.5k of pull-up resistance to +5V.

I think I will try and rig it up to an Arduino and try and read out the contents and compare to the above downloaded file. That file has a 1994 copyright message in, same as the label on my ROM, so it's likely to be the same version.

NJRoadfan wrote on Today, 01:58:

Symptoms are consistent with a cold solder joint on a memory chip. Might be worth a closer inspection for damage.

I have already been over all the solder joints on all the surface mount chips (the GD5429 and the RAM are the only SMD parts on the board) with a magnifier and pushed on them with tweezers to see if there's anything loose, but I didn't find anything bad. I suppose I could try re-flowing all the solder joints, but I'll try other things first.