|Sound's like you have started to diag it.|
List all symptoms and then formulate what might cause is good.
Could use voltmeter to see power supply voltages. Could start and go to bios. Sometimes they offer various diag tools like temp and voltages.
Look for fans running too. Look for heavy dust and debris.
Might try a linux live cd to see if it fails also. A barts with correct video drivers would be better to decide software from hardware.
"Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10