starts with GPU,after some minutes restarts automatically with on-board

[Pelepek1]

So this is my story ...
First my specs:

AMD fx 6300 six core 3.50ghz
8gb of RAM
Windows 10 64
Gtx 660 zotac
Gx bronze 650w
Motherboard: ga-78lmt-s2
Hd 500gb

sorry for the bad writing,not my first language

This system worked for me perfectly for 3 years.
Simplifying everything that happened:

Everything normal with the PC, then I turned it off, restarting it, it started on the on-board GPU, not the 660, strange,i restarted, it returned to normal, then when I turn it off and restarted, the same thing happens, but now it did not detected my GPU, I looked around that several other people had this problem, but no solution worked, my temperatures were always normal, my pcu was with normal 12v, my GPU fans worked,unplugged plugged everything.
After many reboots the computer restarted with the GPU, but sometimes he rebooted the computer in the login or after a few minutes or seconds on the desktop, returning to the on-board GPU, I disabled the automatic restart, but even so, the blue screen did not appear. They said that it could be a hardware problem, my computer is very clean, so it's not dirt, after some restarting my PC give me the option to reset, i accepted, it started with the GPU, but after some 7 minutes
  It restarts automatically with the on-board for my pain, i don't have other PC to test the GPU or friends that have a good PC. With the on-board graphics the PC don't restart for my fear, so that it, it starts with the GPU, but after Some minutes or seconds it restart automatically with the on-board not recognizing the GPU, sorry for the long text, thx in advance.
update:after the last windows update,the GPU worked fine for hours,but while i was watching a movie ,the display freeze,after a restart it start only with the on-board.

 

[dontlistentome]

I'm not clear on one point: you say it started on the onboard GPU, do you mean you removed the monitor cable from the videocard output and connected it to the monitor? Or are you saying you have both the videocard and the onboard video connected to the monitor?

 

[Pelepek1]

I'm not clear on one point: you say it started on the onboard GPU, do you mean you removed the monitor cable from the videocard output and connected it to the monitor? Or are you saying you have both the videocard and the onboard video connected to the monitor?

i turn the pc on,the 660 send the signals to the monitor normally,after some minutes or seconds the 660 stops sending signals to the monitor ,them i plug the monitor to the onboard [PC still on] and it works,always that.
hope that answer your question,i did not understand well.

 

[Pelepek1]

update:after the last windows update,the GPU worked fine for hours,but while i was watching a movie ,the display freeze,after a restart it start only with the on-board.

 

[Pelepek1]

no one?