Computer Shutdown during Gaming

Try reapplying thermal paste to the CPU. If if its doing it under heavy loads, it could also be the GPU overheating. Try replacing the new GPU with the old one, (New CPU and old GPU) to see if it does the same problem. this will rule out one of the options.

During heavy gaming it shuts off after about 15 min. No warnings or alarms before or after.
here is your clue .

you need to change your PSU .
Open your cabinet [ with pc on ] lightly knock your PSU , see what happens
if your system is randomly shutting down without any prior warning then it must be your PSU

I think there is nothing wrong with your thermal paste , if this were the case your PC would have " Restarted " this happens because of overheating .

almost all modern day motherboards have a feature in them which will shut down the system if Power supply goes wrong .

sorry for Bad English .

Update: Turns out AIDA64 stinks at reporting accurate temps. I also must not have saved the BIOS settings with the alarm on. Switched to CoreTemp, made sure the BIOS was saved, and it is definitely the CPU overheating.

After reinstalling the CM cooler (same behavior), cooler master said the cooler is bad. I guess the bottom of the CM cooler is not flat, or something is wrong with the heat pipes. Back to newegg with you, POS......

I reinstalled the stock cooler and no issues. Except for the airplane noise under my desk.....

I'm getting a AC Freezer A30 instead. I may have to pop the heatsink off the closest RAM stick, but it will be worth it.

I just installed an AC XTREME 7970 on my HD6950. Other than the crappy directions and length of down time to do the install, it is awesome. It took 24 degrees C off my GPU temps, AND you can't hear it!!!! Had to stick my hand down there to see if the fans were running.

Their tech support was really helpful in figuring out the best placement for the glue on heatsinks. The directions only show 7970 placement. Downside is they are in Switzerland, so communication is not the fastest.....

SO this solved your problem ?
so i must be wrong , but i had the same problem and need to change my PSU ,

yep. I checked the psu. it's ok.