Sounds like a GPU issue. Could be a motherboard issue. I would try to start eliminating possibilities by using just one GPU in each PCI slot, then the other GPU in each slot, etc. Could also be a RAM issue, so I would also try to eliminate that as a possibility by making sure all your sticks of ram are good, what do you have 3x2gb sticks? If so that's likely it, as most DIMM slots these days are supposed to utilize a PAIR of memory slots. So if you're using 3/4 slots, you might find a 4th stick, or the removal of the odd stick out will solve it. Definitely start eliminating possibility by ensuring the integrity of the GPUs separately and each RAM stick.