I would suggest taking a look in Windows event viewer. Also, check what is going on in resource manager, see if anything is hitting 100%. Be sure that you have the latest versions of your drivers and your OS. Check what background process are running. A malware scan wouldn't hurt ;) Another thing to try would be a brand specific diagnostics tool on your hard drive. You can post the SMART status here after you have done the extended test. It might be a long shot, but is worth a try ;). There are third-party software tools, however the manufacturer's usually provide the most accurate results.
P.S. Test your PSU on a different machine if you can. There could be a problem with it, and PSUs have the unpleasant habit of going out of order with no warning or obvious symptoms.
Keep me posted,