There are several possible problems. I Do know for sure that the motherboard you are using has weak power delivery for that CPU and during intense gaming it is liable to overheat the motherboard and throttle your CPU down. running with an open case can help, as can using a zip tie to get a case fan blowing directly on the power delivery components (VRMs) between the CPU socket and the rear I/O shield. When that is done we can look at other issues.
Has the computer ever performed well, or is this a new situation or is this the first time you have run this game?