It most probably is due to overheating, its amazing how much dust will accrue over the years...
Get a can of spray duster, unplug the machine, take it outside (in dry weather of course) and dust it liberally.
You may at this point also wish to take out and re-seat the memory as the contacts oxidise also which can give you a BSOD also.
After that see how she goes....
p.s. Most computer shops PCWorld, Curry's (Or Comp USA in the states) will stock spray duster.