Driver Problem???

I have been having a few problems recently with my 2nd BFG GTX280OC (RMA'd
1st) and when booting the system seems to hang at the point the grafix card
boots up in windows Vista. The card has been benchmarked with Vantage but
only the default settings work to give a good result. 13000 default and
5279 overclocked. Hence I always run it at default settings but recently it
has got soo bad that the screen goes black and recovers a few minutes later
with the message saying the driver stopped responding. I have tried various
updated drivers and even vainly tried going back to old original drivers but
all the same. Is my GTX on its way out?
I have a Hiper 880W PSU, an IP43 mobo and a water-cooling loop all inside an
Antec 900 case so cooling shouldn't be a problem (GTX still on stock
air-cooling).
I am using Vista64 with 8GB DDR2 800 and only 1 HDD and 2 Optical drives,
surely this cant be a PSU.
Any advice will be gratefully appreciated.

Robin