I have the same issue with the following configuration:
P6N platinum, MSI 8800GT 512MB, 2GB DDR2 800 Kingston Value, C2D E6700, Thermaltake purepower 600W, ASUS silent square EVO. Last week I had a Gigabyte 8800GTS 320MB and everything was OK until the 8800GT.
After so many tries I can make it...