Plutonium

Hi,
I started using plutonium pretty recently, and my issue is i played zombies with a couple of friends and everything was ok my fps didnt seem to drop or to go crazy, later we decided to go a 2x2 between us, and then something happened that was i was still with the same settings but my fps couldnt stay at 60 or even more, they were as low as 10 fps or lower at some instances, and didnt wanted to go higher as 30 fps