Does this happen with every game you play? If not then its the game at fault. When the shop had your pc, did they try to play a game on it? If so, did it run ok? If I was you, I'm assuming the pc is still under warrenty, I would ring the shop and explain whats occurring. The only other thing you can do is to :
A. Make sure all drivers are up to date including chipset, graphics and sound.
B. Ensure all power cables are fitted and none have come loose.
C. Check overheating possibilities (which you've done).
D.Re-set the 'Bios' to default settings.
E.Check Ram by only using one stick at a time and trying to play a game. One stick may be faulty.
F.Download Everest here:
|MG| EVEREST Free Edition 2.20 and use it to moniter temps ect.
If you have done all of the above, you then have to start thinking in the terms of something is faulty hardware wise. As I mention above, you say the shop has had it back but did they run the proper tests?? You must tell them exactly whats happening and when. The pc can't run ok for them and not ok for you...
Use that warrenty man and get on the phone!!