[CA] [CA11] Drift and auto helm failure

[HerbandSplice]

Seems that I have multiple issues developing with steering on my new-to-me 2006 Nauticat. The first issue I noticed was drift to port (constantly having to slowly turn upper helm to starboard to maintain a straight course). Then two trips later, on autopilot, the boat suddenly initiates a spin, and autopilot indicates a fail. After re-starting the nav system and on autopilot again for an hour, another sudden spin of the boat and auto pilot failure. I reverted to manual helms, and found that the lower helm exhibited slippage (6 rotations of the wheel) before I got engagement with the rudder, but then another 6 rotations in the opposite direction to engage the rudder for the opposite tack. Fortunately, the upper helm did not exhibit air in the line so continued on to my destination manually. At the dock, I isolated my auto helm by closing valves to all three lines running to it, which had the effect of reducing the drift I had been experiencing on my upper helm. The lower helm continued to exhibit air in the line. I have yet to bleed the air from my lines.
My question is: Could air in the line be the cause of my auto helm failure, and my drift seen in the upper helm? Confusing is that I see minimal leaked oil externally, and am at a loss where this air would have come from (at this age of system - 2006), and the fill line has been kept full.
The system is a Raymarine ST6001+ autopilot w/type 2 drive.
Your advice would be appreciated.
thank you.
Herb