I don't know if this is relevant to your situation regarding the "squirrely" behaviour, but I was experiencing a similar behaviour with one of my add on aircraft and Accu-Feel, to the point that I turned Accu-Feel off. It turned out that the aircraft was exceeding the max indicated airspeed setting so she started snaking and gettin all outa shape. When I upped this to a more realistic value (it was set way too low) the problem resolved itself.
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