Hello keis3311, welcome to PCHF.
Sounds like overheating to me keis. When the card reaches it's maximum temperature threshold, it will "Throttleback" and reduce it's clock speed. You should notice that the problem returns to normal after coming out of the game for 5-10 minutes (without restarting).
Use Speedfan to check your system temperatures:
Download SpeedFan - Access temperature sensor in your computer
Use NTune to check your GPU temperature:
NVIDIA nTune
You can monitor them in the background whilst playing the game. Check your temps before, and then [ALT]+[TAB] or end the game and check them again.