Gainward GTX 260 Problems?

Hi, I recently purchased two Gainward GTX 260's GS for my system:

Evga 790i SLi FTW
Q9650 3.0Ghz
CORSAIR DDR3 TW3X4G1600C9D 4GB RAM
Windows XP SP3

Nothing on the unit has been O/C'd as yet and i was planning to start after installing the mobo yesterday. However, (even prior to the mobo installation) I am having some issues with the graphics cards on NFS: SHIFT. This game is probably the most graphically taxing game I own and what happens is that 10/15mins into game play the screen will go Black on/off about 3 or 4 times and then the image distorts creating artefacts, green and pink pixels, vertical lines as it switches itself on/off as if trying to change resolution.

GPUz doesn't show either the cards GPU temp go beyond 79c & CPUz shows the CPU cores around 52c. I always put both fans on 100% when playing NFS Shift & yesterday took out the sound card and using the mobo inbuilt just to give the cards more breathing space (but problem still persists).

I have also uninstalled the drivers (using driver sweep) and reinstalled the latest drivers from Nvidia and then the beta drivers I found posted on Gainward site from only a week ago. Still the same problem.

I am not sure what the problem is as when I switch off the pc everything returns back to normal. When I run Benchmark 3D, no issues arise there. Is it a fault with the card/game/cpu/ram/drivers??? Any help/advice would be most welcome. Thanks!

I have only played GT Legends, GTR2, Race 07 and Flight Simulator X on the system with no issues. All are older games however I may do some testing with FSX tonight and max out the graphics and see what happens. Previously had a 8800GT and left the graphic settings pretty much the same on installing the GTX 260's.

I am going to get a copy of ARMA2 and again put the settings up and see what happens. I think i may need to run the cards individually & in SLi to see if its a particular card or the Sli bridge.