I would suggest downloading the newest driver from
HERE and when installing tick the box for 'perform a clean install' this will uninstall all your settings and drivers and reinstall from scratch. They may have installed incorrectly or you may have edited settings incorrectly.
Have you overclocked your graphics card? How is it connected to your board + psu?
Take the side panel off your case when it's running and see if the card is running hot.
Have you got a spare PSU you good use to check that? If not wait for darkmantis to post the checks needed on PSU + RAM, many factors could cause this.
Might be worth doing a MemTest86+ on all your ram to check it.
How long have you been running this setup before encountering problems?