Tru Trak Digiflight II pitch problem

[fb9jann]

The Tru Trak Digiflight II autopilot that's installed in an Arion Lightning operates fine in its lateral and vertical (Alt Hold) functions but when the GPS commands the autopilot to turn to a new heading at a waypoint, even if only a few degrees, the pitch servo starts to put the aircraft into a dive. I have to pull back on the stick, against the servo, to keep the nose at the correct pitch. Once it is back on an even keel I release the stick and the Alt Hold performs fine. Any ideas what may be causing this?