Graphics card keeps overheating

ive been haveing a problem were my screen turns black while im playing games. I thought this was a problem with my processor so i bought thermal paste, My temperatures went down to about 40idle 50ish while running something. Before they would be going up as high as 70 Celsius. i have a q9300

But for some reason my screen will go black even while watching youtube videos. My GPU temps go up as high as 70-80 while idle. What exactly is the problem and would it be possible to add some thermal paste to my GPU?
 
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