Following up, today I did a long flight in a different plane and had the wild swings of heading right off the bat. But all else was spot on, location tracked perfectly with my ownship ADS-B target. I was using a STRATUX that had its own GPS and AHRS, but a woefully old software build. When I disabled WiFi and used the integrated iPad GPS all was well.
Interestingly, my previous flight without the heading issue was with an ilevil AW3. I will try it in yet another plane, with a STRATUX using a current software load and see what happens.
RV-6A bought flying