I had that problem with a revision 3.0 A3640. The machine would get slower and slower until it would lock up. It wasn't overheating but as it warmed up, the cpu board wasn't reading a zorro signal properly. More heat == more resistance. The problem would show up in summer but the machine was ok in winter. If I removed all zorro cards, the problem would go away. It would also stop if I removed the case cover and put a fan blowing on the cpu board. This was on an A4000D.
