may be heat is the issue cause cpu and gpu downgrades there performance as temperature rises . . check your temperature on start and then when fps go down
You should post your whole build first, then we could see if there's any obvious issue. I would suggest that you play the game and follow the temperatures, and listen for any obvious noise if any fans go spinning crazy to try cool something down.
There should be no bottleneck. I would check your temps.
+1 for this.
If it starts out high and then drops it sounds like CPU and GPU heat saturation inside your case. You may need to improve air circulation with higher RPM side and top vent fans.
i'm kind of a noob if it comes to oc and checking heat what program do you recommand for checking the heat of my cpu and if my cpu *C is to high were can i turn up the fan speed ...