SureAI

Using 2 cores it played perfect for many hours and I was already glad that my problem solved, but then monitor signal missed again and I had to reboot pc. I dont know what to do

Have you ever tested how it affects the hardware overclocking undo? Did you also overclock the graphics card?
I still believe it's a hardware problem. It is possible that problems only affect some programs.
Alternatively you can try to reduce the overclocking.

edit: With your system, you do not really have to overclock for 3-4 FPS more ...
The Skyrim engine is limited to 60 FPS. You can set this in your graphic driver. Maybe this helps...

GPU not overclocked, stock frequences and power limir. I can try reduce CPU frequence but I think it's not necessary. If its hardware problem it can be or psu voltage problem or bad connect (6 pin or pci-e). This is definitely not a psu problem, because I measured the voltage with a multimeter, its withinin normal limits. Will try move card to another pci slot and will change power cable.

Please report. We have never had this problem...

And again: disable your overclocking. It is a coincidence that the problem so far only occured Enderal.