when I come out of a turn Slerp does not always straighten my aircraft out

I’m writing a Star Fox type of game in 3d. I’m trying to get the aircraft to return to 0 rotation after a turn.
I’m new to Unity and game programming. I am a very seasoned c# developer.
What seems to happen is the aircraft returns back to 0 most of the time but then at some point the
rotation will not return to 0.
I’m attaching pictures to show the problem. (Please read picture descriptions).

157657-problem-didnt-return-to-0-after-bank.png

Here are a few more pictures157659-turning-left-example.png