I've been through re calibration a couple of times, but our autopilot has become unreliable at random times with the displayed instrument heading way out of sink with the binnacle compass, and the boat wandering off course. I'm beginning to think something might be going on with the flux gate compass. It seems Raymarine no
longer services this 20+ year old model, or makes a replacement, and the only products available are used on ebay. Buying used doesn't seem a good idea, especially with no mention of it being reconditioned. 

1) can anyone help diagnose to verify this is the source of trouble?

2) There appear to be a couple sources selling OEM internal flux gate workings for replacement. Has anybody gone this route?

Thanks for your input