If i were you i would do the following:
Run memtest..just to check that the ram is fine.
Then i would do a clear cmos and then update BIOS (even if you have teh latest one)
Go into windows and put all teh settings to default
Then reinstall windows and install the drivers form the website in the following order (this is important): Chipset audio video lan then anything else.
Now test the system. Test as you already have with a few games see what happens. If you are still getting the issue, you are going to have to cross test the components, try a differnt CPU, PSU, RAM even try putting a graphics card in. This way you can isolate whcih is teh faulty component and then replace it.
Hope this helps.