|You state the original drive did not work at Best Buy, but have you verified the 250GB one is good? How old is the 250GB drive|
CPU cooling is important to keep the system from shutting down randomly or throttling when getting too warm. blowing it out is a quick way to check for dust buildup and system operation. If you get lots of dust and the blowout helps then the best cleaning would be to get to the fan and remove all the dust.
It appears you also might need to check your memory for errors with memtest 86 or another program.