I suspect that your CPU is spiking and causing the frame rate drops. But it's hard to perfectly correlate. You may try running perfmon or some other monitoring tool to record the CPU load and see if any of your cores are flatlining at 100 while your fps drops. CPU load way say 70%, but that's an aggregate. You might have 100% on two cores and 40% on the other two, which would be 70% overall load.