Nav mode stopping 2/3 of the way in and wobble

[WinterHuntsman 38]

So this has happened for the 4th flight for me and results in me having to use the heading selector to finish flights. During flight the first 2/3 of the flight with nav mode in for autopilot everything is fine and working as intended. Right around the 2/3rd mark in the flight my plane develops a wobble like the nose moves left and right a lot and the back of the plane does the opposite. It’s like a fish swimming motion but not dramatic but noticeable. At the same time nav mode while active no longer will do not follow any of the autopilot and flight plan inputs. Switching to heading fixes it and stops the wobble as well as allowing me to guide the plane on somewhat autopilot. However if I turn back on nav mode to problem returns.

So with nothing seeming to be wrong at all for so much of these flights until the last third that I don’t know what could be causing this to consistently happen. If anyone has any ideas please let me know, I have read through the manual and everything looks right to me, but I still don’t know why this problem keeps happening on the last 3rd of my flight.

 

Edit- Ok someone just told me on another forum the nav problem could be caused by discontinuity in my flight plan and since I have been importing plans, I’ll have to type it in manually and see if it fixes the nav issue. The wobble is still something unknown as it happens at the same time the other problem occurred and I am using the modern flight model.

[Puukka 28]

I've recorded a video regarding the wobble. There are more possibilities NAV mode disengages as example switching the NAV source of the PFD at the side panel.

Regards, Herbert