Yeah it sounds like ur gfxcard in all fairness. If its overheating or isnt getting sufficient power it may be turning iteself off. Even a momentary spike on the heat of the card can cause that. My old card when it was overheating would attempt to switch to a different resolution but my monitor didnt support that so my screen went into standby. Check the fan on the gfxcard and see if theres any dust or other crap in there. Otherwise i have no idea

edit:- actually i read somewhere recently that 6600's are tempermental about traces of old drivers still hanging around. Run drivercleaner pro and see if theres old drivers hanging around (like if u upgraded from an older card and didnt remove the driver)