Rogo
You need to calibrate the steer center setting to stop it from drifting off center providing the tach generator is not generating noise in the left turn circuit causing it to drift.
There is a calibration routine in the steering section of the manual for this and other steering functions.
One other thing you may want to check is the hour meter display on the BDI meter and see if it is displaying any codes when it is driving in slow mode.
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