Τhe problem came back again.
Let me help you remember the problem and all the steps i have done :
Problem No1 : PC suddenly freezes at random for 1 minute and then PC shuts down
without BSOD.
1st solution i tried : Format and reinstall Windows and update all the drivers.
Result : Problem not solved.
2nd solution i tried: I removed all the external usb components except the mouse and the keyboard in case they caused a conflict.
Result : Problem not solved.
3rd solution i tried : Replaced the mouse and the keyboard in case they were defective.
Result : Problem not solved.
4th solution i tried : Ran a program that tests all the hardware ( i don't remember the name of that problem but it was one that was suggested here). No problem found by the program. I also ran windows memory diagnostics ( not memtest).
Result : Problem not solved.
5th solution i tried : Replaced the PSU.
Result : Problem solved for almost 3 weeks. Then it came back again.
6th solution i tried : Replaced the MB.
Result : Problem solved for almost 1 month.
And then something happened that got me again frustrated. The problem No1 was solved but another problem appeared that also brought back the problem No1.
Problem No2 : Even if the boot order was 1) My internal hard disk, 2) CDRom , 3) usb disk, the PC couldn't boot Windows when an external hard drive was connected to the PC before boot. If an external hard drive was connected then the BIOS was forgetting the boot order i had set and was putting by its own the external hard drive as the 1st boot device.
1st solution i tried : I went into BIOS and in boot order i disabled all except my internal hard disk.
Result A: Problem No 2 solved!
Result B : After the fix of problem No 2 , problem No 1 came back!!!
After all these, i tried to understand what the hell is happening with this PC. So many solutions, so many changes, so many replacements for 1 year, so many hours spent and all for nothing!
So today, i thought of something that had never crossed my mind. I thought : Why BIOS forgets the boot order? Is it maybe because it looses sometimes my internal hard disk , so it goes to the next boot device ? Why for 1 month everything works great and then the problem of sudden freezes and shutdowns happens again? Is it because after the 5th and 6th solution i had my hard disk formatted with new clean install of Windows 8.1, Programs and drivers?
In conclusion : Is my internal hard disk the one that is failing when it is loaded with many data and causes all the problems? After all the internal hard drive is one of the 3 hardware components that i haven't replaced since the problem occured. The other two are the RAM and the VGA.