What to do with unexpected shut down and BIOS corruption

Hi guys,

My PC is shutting down unexpectedly. Sometimes do not restart. Last two times tries to start again, but couple of times fails. Starts for a few seconds and stops again. Again and again after show me message that BIOS is corrupted and recovers it. After Windows starts normal.
I did these things – cleaned components, changed cooling paste on the CPU and scanned for viruses with Microsoft Security Essential. After went to the service and tested power supply. They told me everything is ok. Tested PC with Prime95 all night and nothing. Before three hours problem appeared again.
My Motherboard is GIGABYTE 990FXA-UD3,
CPU – AMD FX 4300 Quad Core
8GB RAM
GPU – NVIDIA GeForce GTX 650
Windows 7 Ultimate

Give me advice, please?

Thanks in advance!

Have you thought about changing your CMOS battery? If it's failing, it might corrupt your BIOS with it's 'flickering' power.

I have not thought to change CMOS battery. Is it possible unexpectedshut downing to be reason from this battery?

Have you tried resetting BIOS to defaults, after taking note of SATA controller settings which should ideally be AHCI.

I'd also look for latest BIOS update and install it, if necessary reflash the current version to see if that resolves it. I prefer to do these from Windows although others think bootable media is safest for flashing BIOS. Whichever you use remember that it cannot be allowed to lose power during flashing.

If you run with a boot disk with HD disabled or unplugged, do the problems persist? Peppermint3 - Create Live CD/DVD/USB To Use For Emergency Backup - Windows 7 Forums

What is happened after two weeks:
The problem - unexpected shut downs - were very often. The PC couldn't start - tried but just started for few seconds. When switched to other bios worked more, but after again switched off.
Replaced the power supply. No effect.
Tested with RAM, power supply, CPU and motherboard. Flashed and updated BIOS. Problem stayed.
Changed slots of RAM. Worked for a day, and the problem happened again.
Went to the seller. Bought a new mother board - the same. Worked all the time, but yesterday I saw end of the problem in the morning. Screen with warning that my hibernate.sys is corrupted. This is boot screen. Before it usually the PC shut downs, after updated bios, trying to start, fails, tried to recover windows, again fails, and after shows that screen.
For two days - this is just one problem.
Everything is tested - memetes86 - no errors, prime 95 - nothing.
I didn't reinstall windows. Do you thing that could be a reason for the problem?
Or what else?

Thanks in advance and Happy New Year!

Did you run with a boot disk as suggested to see if problem persists, which would rule out the OS and possibly HD? Do this with and without the hard drive connected.

You can do the Clean Reinstall Factory OEM but I would wipe the HD of all code to make sure nothing remains that can interfere. The "Clean" command to do this is detailed in Overcoming Installation Failures

I tries without HD and problem persists. Strange that from two days there is no problem. With new Mobo I saw once screen that hibernate.sys is damaged. I am not sure if before that there was a unexpected shut down. But with old there is after shut down and own tries to start.

Try turning off Hibernate, reboot and turn it back on: Hibernate - Enable or Disable - Windows 7 Forums

OK, I'll try. Strange, but now in hibernate, in Advanced power options shows me - hibernate - never. But I'll do like tutorial shows.

I did the tutorial. It's proceed to be ok - not shut downs. But before to start shut downs very often it was a 10 days period without problem. After - very often. I am interesting if it is possible to be a virus could damage bios. After MOBO switches other bios to next time when the PC is infected again. Very strange problem. I can't stop thinking about it.