i'll go with leongrado ... it is most probably an overheating problem
Check your gpu temp using msi afterburner :http://downloads.guru3d.com/-MSI-Afterburner-2.2.0-Beta-9-download-2816.html
If the temp rises a lot , then it is an overheating problem.
Check your cpu for blocked fans ... if u can do it yourself .. do it .. otherwise get your processor and gpu fans checked ...
It also could be that your cpu needs an extra fan for the amount of heat it generates. In that case you'll have to buy a new aftermarket fan ..