Sounds like your CPU is being throttled due to high temperatures. The Real Temp program can help you monitor your CPU temperatures and the CPU-Z program can help you monitor your CPU speed. At the time you experience the FPS drop, alt+tab to your desktop and look at the readings for CPU-Z and Real Temp. If your CPU is showing speeds below 1.5 - 2.0Ghz then you probably need to clean out your computer case. Buy some canned air and use the crevice tool on your vacuum to expel then vacuum the dust and debris.

If that isn't the cause of your fps drop, then it could be your GPU being throttled for thermal issues. MSI Afterburner can provide you with GPU temperatures if that is the case. Good luck.