Windows 7 professional random reboots

I have a prebuilt HP computer that I have upgraded the video card and PSU computer is only about a year old. Just recently I have been getting random shutdown and reboot, there is no blue screen or error when it reboots. It does it at random times watching movie, browsing internet, playing games. When it reboots it goes through the POST screen with no errors and I didn't hear any beeps, instead of loading windows it loads a black screen that states at the top "ENTER BOOT IMAGE: 0.." you can enter the numbers 0,1,2,3; but when you type a number or just wait it locks up and have to hard restart to get it to reboot. It does this in a continuous loop.
This is the steps I have tried:
1. Repair using the windows 7 disk
2. restore to a time before I had troubles.
3. Updating all drivers(after the problem first happened)
4. Virus Scan the entire system all files

the only thing that does work and gets it to reboot and act like nothing ever happened
from the command prompt in the win 7 disk
type the three following commands:
bootrec.exe /fixmbr
bootrec.exe /fixboot
bootrec.exe /RebuildBcd
after those three i close and restart and it works fine. This problem has been troubling me only seems to be once a day for the past 4 days.
I would like to fix this without a clean Install, and please tell me why it works for about a day then happens again.

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