Giant wiper motor servo problems

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Hello al I’m new here !
I am trying to build a large scale servo For a large RC project
From a wiper motor modified closed loop
A tower pro 996 servo wires removed from original motor to signal
My robot market place HV simple H bridge using a simple linkage
Back to the servo for feed back.
It does work ( kind of ) but I have feed back issues causing extreme left/ right polarity switching to the H Bridge
Which I don’t under stand what I’m missing !
If anybody could give some advise it would be greatly appreciated