It's possible that this is hardware related. Bad memory stick or overheating can cause the symptoms you describe. You don't say how much memory you have, but if you can try removing the modules, testing again and then re-instyalling them.
Check that your fans are all operating correctly. If this all fails to correct the problems consider borrowing a PSU if you can and try that.
If that fails to sort you out, then a re-install is probably your only option.
Whilst you are in the case, remove the graphics card, clean the fan or fans and reseat it in its slot.
Clean any dust from the ventilation areas as well. Good luck.