i had something similar with a clients PC... i found out, this PC was extremly infected by viruses... esp with a very extreme and destroying virus called "Virut" where i had no change to remove this virus with remover tools nor by hand nor by any antivirus software nor antispy tools - even in safe mode...
Just check out following things:
1. Open your PC case and check if the CPU and GPU coolers are free of dust...
2. Boot your PC and check TaskMen (tasks and if there are any tasks eating a lot of CPU ussage)
3. Try to remove all unneeded AutoRuns and hidden programs starting when booting windows.
If this all fails, try following steps:
1. Download SmitFraudFix and run this program in safe mode...
2. Download Malware AntiSpyware and check your PC...
If all steps fails, you should do following:
1. Turn off PC (disconnect powercable), remove all kind of hardware extensions like TV Card, Modem Card, CD/DVD, so you have at least only the Mainboard + CPU + RAM, GfxCard, Boot HDD...
Check out if the problems with SOD still exist... if so, it could be a software problem or still a hardware problem like an error in RAM module or Error on the HDD or anything else....
What you have to do, is to check all possibilities (software and then hardware) step by step...
Good Luck...