Do you have video shadow and/or palette snoop options in your BIOS? Try disabling both. Are you able to apply more modest memory timings if possible? See if that makes a difference. I wonder if it's some kind of memory corruption by DMA transfers.

Thanks for the reply! I disabled video shadow, there is no palette snoop option. The highest figures are the most conservative aren't they? Changing these options seems to reduce the instance of it occurring somewhat, but that may be due to race conditions, since changing the system clock speed or disabling cache can have the same effect.

I'm not sure what things you can set but often higher numbers = slower when it comes to RAM. But I understand it doesn't make much of a difference. Do you have another VLB card you can test with and which card are you currently using?

Tried both cirrus logic and S3 cards, and both have the same results. The problem goes away with an ISA svga card.

Have tried both VLB jumpers (1WS and >33Mhz), but neither make any difference

I'm afraid it's the infamous VLB instablility, seeing you already swapped cards and tried different settings. Can you move the VLB card as close as possible to the CPU and the soundcard as far away as possible to see if that has any effect? Hopefully one of the VLB experts on Vogons can advise you more.