or i could be a software related thing.
Try in safe mode, press F8 when i turns on til you get the boot menu list. See if it happens in there. If it does its probably heat. If it doesn't its probably either drivers of software causing the problem.
If it doesn't, start-run type "msconfig" and go onto "startup" and de-select all of the items.
Try again in normal windows. Then go back into msconfig and start ticking back on, one by one, til it happens again.
Or if its drivers.... Control panel-Device manager - delete the probable cause - but be careful.....