What power supply are you using. The overclock on the processor takes alot of power. So does the 2 cards in sli. It could just be that with your old monitor the system wasn't being pushed hard enough or rather the power supply wasn't being pushed hard enough to cause system instability. Only thing i can see that would cause this problem if the computer was working perfectly before your new monitor. Switching to a lower resolution should have caused the problems to stop if this indeed was the case. The computer doesn't really see a new monitor all it sees is a device that supports a higher resolution. Did you open the case and reassemble the computer. Make sure the 6 pin connectors are connected to each card. I think it was 6 pin for your cards. Switching to a higher resolution stresses pretty much every part of the computer except for maybe the sound card lol. Anyone of them could have been slightly defective and worked fine till you switched to a higher more demanding resolution. Didn't mean to alarm you but thats kind of the worst case scenario. Some ideas for isolating the problem would be to disable sli. Your frame rates would be cut in half but 250/2 is still 125 perfectly playable. If the problems stops then you know the graphics card is drawing too much power from the power supply or one of your cards is bad. You mentioned driver issues. You said that you formatted windows so unless you downloaded a corrupted install drivers should be fine.
My 9800GX2 had simmilar issues at that res with all 180 series drivers. Using 181.20 WHQL cleared that up though (except when using hardware accelearation with flash player in fullscreen...still figuring that one out...).