if your pc is freezing maybe you should wrap it up in a blanket and give it a warm drink

sorry couldn't resist.... I agree with Und3rtak3r... it sounds like something is getting hot inside the case - have you tried taking the case off, letting it cool down for say 20mins and then trying again? If this produces some benefit then I'd reckon you need more cooling. Another thought - is it particularly dusty inside the case... you might want to try blowing some of the dust off if it is.

NB use a can of compressed air rather than blowing with your own lungs

Z