Sounds like a over heat problem. Are you checking your temps? Another thing you could do is open up your case and blow all the dust out of it first to see if that doesn't help it.
If that doesn't help or you already have done that, then look on the net for MSI Afterburner and down load it and check your temps and your fps and a few other things.
This way you can for sure know if it is your temps. I would link you to it but I am on my laptop and I don't have saved or in my favorites. Get back to us and we will go from here. Good luck.