Centering your Navomatic autopilot

[n86121]

It sounds like to fly level w autopilot,
you are offsetting two errors,
using the aircraft trim to balance an offset in the autopilot elevator.

Thus, when you turn on altitude hold,
it tries to go 'back' to where it wants to be,
which puts you into a climb or dive.

I would re-zero the elevator servo and that might straighten it all out.

If at some point the cables were re-rigged, even a little bit,
that would introduce an offset into the autopilot