did you try installing a previous version of driver and then updating it to the newest? many of my Graphics card related problems were sorted by that. Also, it would be a good idea to move up to a 600W PSU.

A stupid question (actually very relevant when addressing noobs, you'd be surprised!).. are all connection accurate? (you don't have to answer that :p)

Have you tweaked you default settings? if so, try settin them default and then install.