Just for completeness I thought I should finish off this thread by relating the resolution to the problem we were having.
It ultimately turned out to be a wiring problem where one of the 4 wires to each servo had snapped at the crimp pin in each servo connector. Entirely our fault, not TruTrak's. Indeed a shout out to TruTrak is due, since they were extremely helpful in the troubleshooting process, and even sent us a new header unit to try (despite I think believing themselves that it was unlikely to solve the problem). We had to eat humble pie after proving them 100% right! In our defence it was a bizarre coincidence to have two faults which had identical symptoms causing us to assume that the problem was somewhere common to both servos.
The performance of the autopilot is outstanding - we went up for two extensive flight tests yesterday in moderate turbulence and it handled everything we threw at it with no fuss, no over-controlling, no hunting and no unpleasant characteristics at all. The only problem we found was that we didn't get a trim annunciation, which is likely due to another broken wire from pitch servo back to the header unit. We'll fix that next time we have the baggage wall off for inspection I suppose.
Andy @ EGLM/EGTB
RV-6 (Shared, Bought Flying)
Building RV-7 #74424