Vista - system freezes when using sli for games

My motherboard is asus P5n-E sli..Due core ,,6 gig ocz sli ram..I have 2 9600 gso 756 ram vid cards ..I play cod world at war and Vanguard mainly..if i use one card the games run great,,When i turn the 2nd card on,the system runs fine ,,But when i launch a game it only takes 30 sec to a min for the games to freeze ,The whole system locks and i have to reboot,,I use vista 64 ,,I have an ultra 850 watt power supply..Everything seems to be in right,,i have switched the cardsaround and they work by them selfs.. Also i ran 2 8500s on the same system and the sli works fine..The chip on the main board is set too 2 cards ,,I am not super up on this stuff,,Any help would be greatly appreciated,,I really wanna use both these cards,,the framerates in vanguard are almost double for the 30 sec it works,,I have alos tried a few diffrent drivers for the cards from nvidia too..

ok guys i updated my bios..no more freeze ups!! man am i happy..Thx guuys

I have the same motherboard... and this problem only started occuring after I was cleaning my system...

This is another factor what I am quite unfamiliar to...

Is this a definate solution?