We are using VCMP's built-in function to check FPS, but seems like the result from the function is not updated that frequent( probably 5 sec once ). I've changed the FPS & ping detector to check once per 10 sec instead of 5 sec..well it doesnt solve your FPS problem but it will give time for the function's result to be updated.
About FPS problem, there could be many reasons that caused it..it could be custom stuff/client-script from server, your computer, VCMP bug/unoptimized or your game setting/modification( you can try playing with fresh install of VC and VCMP, but i cant promise it will solve your problem ). I dont face the problem, so it's a little bit hard for me to find out the reason your FPS is dropping.
You can also try what Brian had said, open your game > Options > Audio Setup > If your 'audio hardware' is 'Rad game tools RSX 3d audio', change it to 'Auto-detect hardware' or something else.