The i5 2500K comes with built in Intel HD 3000 Graphics. The low framerate seems to indicate to me that it may be using the onboard graphics rather than the GPU.
An unstable overclock could cause performance issues. Even if it's not unstable, some games don't play nicely with OCing. For example Battlefield 4 crashes on pretty much any GPU overclock. A 4.2 GHz OC is pretty high for a 2500K.
Actually its not. Most overclockers use 4.5 GHz clock with that CPU, I use 4.4 GHz myself and can tell that it is perfectly stable and I have no problems in coh2. On the other hand coh2 doesn't need that much of performance so it will work perfectly on stock 3.3 GHz too.
About Intel HD you are probably right as i have checked the performance of coh2 on Intel HD 4000 and it was just slightly better than what he gets.
To OP: Tell us what is your motherboard model or just the type of chipset, then we will know if you have to search bios for an disable option for Intel HD or it is disabled by default.