Diagnose computer crash that leaves no trace?

[KHRZ]

Have had 4 crashes, 1st time it took a few weeks before the next, last 3 in the previous 2 days. Screen goes gray with some repeating pattern and noise in speaker. When I turn it on afterwards, no output to screen. First 3 times it worked again correctly after 10-20 minutes, last crash it has been over 15 hours before it worked again. At first I was thinking something overheated as I was playing SC2 each time it crashed, but not so sure anymore, all temperatures are low anyway, no help in changing thermal paste. So I'm thinking some component is just really unstable or something. But not sure how to actually uncover anything about the issue....