Tiny hydraulic leak back past the pistons in either the ram or hydraulic pump. Used to happen to us on our rib
. Stern drive had hydraulic steering
and due to the huge torque from a single
propellor the steering wheel
would slowly need to be turned in one direction more than another. However, it only affected us on long passages in a straight line so ignored it.
For you though it will make a mess of any autopilot
steering system that transmits drive via the ram. The solution would be to use a local truck hydraulic firm to rebuild
the ram and pump.