Ok, done two complete passes now with not a single error. Now I'll add the cards back in.

The issue is not likely a heat issue since the system has run pretty cool since it was set up. It's using on-board video, the cards are for video capture only (PVR). It's also rare that one of the cards (the QAM/ATSC) is used at all.

Normally only a USB keyboard and mouse are plugged in - if at all. This did not change to cause the problem I first reported today. The PS2 thing was connected only to try and get past a point in the Windows install during the repair. It could have been what was causing the memtest errors which I'll know shortly.

BUt if Memtest doesn't fail, I'm back to square one, not knowing what the problem is at all.