This added some new info: He told me the rudder position sensor is a wire-wound resistor, and if it loses contact at ANY point in its range it will produce a drive stop error. He says it only takes a tiny gap or intermittent contact to cause an "open" to trigger the error. Since I have a linear sensor he recommended I get a Teleflex AR4502RM (he said it's far more reliable than the Raymarine linear unit, and that the Raymarine rotary sensor is also more reliable than the linear unit). He said it's not enough to test the position sensor at-rest, that you have to watch the ohmmeter while sweeping the wheel throughout the rudder range and if it goes to zero even for an instant it's faulty.

I'm now thinking maybe that's my issue.... stay tuned.