|Don't guess, take it systematically.|
Install HWMonitor to monitor your internal temperatures to see if it is a thermal problem and where. Post the results at idle, normal use, and maximum use (if applicable, gaming).
Test your memory with Memtest via a bootable CD. Run all tests.
Test your hard drive(s) with the manufacturer's utility. If they are different brands, use Seatools from Seagate as the most universal. Use the longest test.
LIST ALL of your hardware in detail for us to be able to determine if you have any weaknesses or incompatibilities there. Sometimes, it is simply that you have added and upgraded hardware but failed to upgrade your power supply, or cheap-ed out and used one that does not deliver what it claims. When listing hardware, list the brand and model numbers. On power the supply (in addition to the model), list the Amperage on the 12Volt rail(s) as well as the overall wattage.
This is just a start, when we have this information, we probably will need additional details or tests.
You have to be a little bit crazy to keep you from going insane.