Since the temp being reported isn't necessarily accurate and it seems to be an issue which is almost entirely isolated to a single game I'd have to agree with Duvie that it sounds like it's software related, and as he pointed out it's highly unlikely the CPU is causing the lock-ups, so what game is it? Have you installed any/all patches/updates for it? using the latest graphics card drivers? BTW, does it only happen after playing the game for awhile (making heat issues more likely) or does it happen randomly, sometimes shortly after starting play? Did you ever apply AS to the CPU and/or GPU? 
System lock-ups, reboots, and general instability are of course the most difficult to troubleshoot so it could be any of the things suggested until you systematically eliminate all the usual suspects.