Cetrek Auto Pilot Issues

[crh]

We have an 727 - 737 AP with a electronic compass unit and its been on the catamaran since 1989, as spec'd by original owner.

Now having used the boat for 2 years no issues except this time when going from standby to pilot, the unit wants to steer about 30 degrees to port. (starts to turn rudder with in about 5 secs of being engaged.)
The heading before engaging pilot is usually about 10 degrees less than being actually steered by hand.

We can use it by steering some 30 degrees to Stdb before engaging and then letting her turn and fine tune to required course.


So we are seeking any ideas or answers as it's an old but until now reliable unit.

Thanks
Chris