That error is windows' Timeout Detection and Recovery (TDR) triggering.
It is supposed to indicate a hardware fault and reset the card in situations where it fails. It has been known to fire off mistakenly though.
Make sure windows is fully up to date with all drivers and software. This issue has been caused in teh past by creative drivers conflicting with the 3d.
Clean up your registry with something like Ccleaner.
Ensure the cards are not overheating, and if you are overclocking stop. The most common reason TDR goes off is overheating or an unstable overclock.
Does it do this with only one card at a time? It is entirely possible for there to be something wrong with the card as well.