Subsidence FPS (Gothic)

The problem is the following: in the game Gothic 1 when you press any key(any) FPS drops to 22. If you wait 10 seconds, it gradually rises to 62. So when I press the movement button, there are wild brakes. Creepy I hate when in such an old game such a low FPS. By the way, when you move the mouse - the FPS is not sags. PC is: Athlon 3200+, GeForce 8600 GT, 1.5 Gb DDR2.