In the level Control, when you have to shoot all those trigens with the machine gun, the frame rates drop incredibly after a few seconds of shooting with the machine guns and once i stop shooting it is ok again.

Some computers have trouble with lots of fast moving objects. Usually its helicopter blades. What video card do you have?

That part of the game is noticably slower if you turn on CryVision while using the minigun. The combination of the light and shadow effects in the tunnel (including the flaming barrel and it's heat distortion), AI coming into play, thermal vision and gunfire effects (minigun AND Fatboy's rockets) can drop your FPS quite a bit.

The main things that affect your ability to play all parts of the game smoothly are your video card, CPU, and RAM, in that order. It's a bit harder to see, but you can use the flashlight instead with noticable improvement in frame rates.

My system (P4 3.0 CPU, X800XT, 1GB RAM) is plenty good enough to play the game without such problems, but when I made a video of that level using Fraps I noticed the CryVision dropped the FPS a lot.