My FPS is unstable, it variates between 50 and 100 and it changes avary 10-20 seconds which makes my ingame ghost changes speed and that is confusing me and making me fail.
is there any way to limit FPS to lets say stable 40 FPS?

Probably because the game is capped at 60 fps (or at least in the older versions)

If you want to bypass the cap try /opt fixedframerate 0

Else go to Setup->Options->Advanced and tick lower frame rate cap

when i tick lower frame rate cap, game is too slow

If you are playin with shaders off them,may be it will work.

Trey disabling Ray Tracing in the options it makes the game look worse but it helps maintain a stable fps.

Lower graphics until you have a stable framerate, not much else to be said unless you give us more info.

lowering everything doesn't solve the problem.
FPS still variates up and down.

swebbles wanna gtbox?

What GPU & CPU does your machine have?

its a laptop i5 3337U and graphics is integrated HD 4000

i havent play for a year or so, before there was about 50 FPS all the time
now after update FPS increased but variates all the time from 50 to 100

Bacic klopp had the right idea...

but instead say

/opt fixedframerate 60
it will automatically lock it