Belisle

I am having the same problem. After engaging the AP, the plane will start to porpoise after 15 minutes of flight or after effecting a change in direction. Disengaging the AP stops the porpoising but starts a 3-500 fpm climb. Manually trimming the plane using the control wheel restores level flight for another 15 minutes. Pitch and trim gyros were removed pitch overhauled and trim cleaned and re-calibrated. Did not solve the problem. Garmin says problem with AP, maintenance shops says no. Any one have experience fixing this? (201j, GI 275, GTN650xi, KAP150)