Other Things to Consider
Rick's right. Anything magnetic around the fluxgate sensor can cause your autopilot to act eratic, but a bad control head could also do the same. You might check to see if you have a cockpit speaker mounted somewhere near the fluxgate sensor. That will definitely cause problems, and causes the unit to act even more eratic when the radio is on. If this is the problem, you can purchase magnetically shielded speakers that might solve the problem. If there isn't a speaker near the sensor, and you can't find any other metal objects (ferrous metal, which will attract a magnet, not aluminum) near the sensor head, its time to call Raymarine direct. Remember, the metal can be on the other side of a bulkhead and still affect the sensor.