The first thing to do is to open the rig up and blow all the dust out of it. Dust and debris in the various heatsinks and fans, can cause overheating.
While you're at it, make sure that all the fans turn freely and run when the machine is powered up.
And also, while you're doing that, remove the video card and re-insert it. Often, theses sorts of problems are due to the video card shifting in it's socket while the computer is moved around - especially if it had a ride in a car.
Beyond that, we'd need more info. But, if it's not overheating, it could be bad RAM on the video card.
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