If anyone is interested, my problem was resolved by setting parameters to these values:
r.EarlyZPass 1 (also 2 sometimes)
r.EarlyZPassMovable 1 (default is off)
It works only together!
Now FPS is more stable, especially on a laptop.
P.s. On a stationary computer, the problem was also solved by this command r.AmbientOcclusionMaxQuality 0 (or more maybe), but it certainly spoils the graphics quality unlike those commands (ZPass)