Position Error with analog servos

I have analog servos 1500 rev/min
Ball screw 10mm/rev.
Input signal +-7V.
Linear encoder fixed to the table 0.001
Servodrive has lag/delay from reference 18ms.
Outpur has 10V=2^15 discritization.

When I try to run jog to 1mm, I get random movement from 0.3 - 2.5mm (That deppends on feed speed).

Unfortunatly I not use linuxcnc but another control system that was present there.
If somebody has such problem when refited old machine on analog servodrives, could have similar problem.
Anyone has any assumptions why this precision problem have place? How thy possibly could be solved?

WHY not changing to the Linuxcnc LIVE sysztem and solve all your problems in a sweep
your PID is wrong as you use a WIN system no RT you will always have this Problems USE a DOS5 Based Controller with ETH and you are fine

As for realTime:
All modern CNC machines that based on windows are use RealTime Core. It costs money of course.
As for Why I don't change to linuxcnc:
I have good experiance with linuxcnc and I like it very much. But to chouse that controller not I decided. Currently I must use it because I build project for it and made all changes. I don't want to rebuild it.

As for my question.
I think I solved that problem with incresing feed back from tachogenerator. It worked prety fine.

As for my opinion for CNC machines based on windows.
I think they are all depends from windows stability very much.
And as you know win is very unstable. And now I have problem, windows is not loading. I'm almost finished my cnc machine and it stacked.
It's realy very bad situation with reinstalling all this payeble software.

After this I like linuxcnc better in hundred times!