Exactly similar symptoms in my old L-Nav.
I had to relight at a contest once because of it. In frustration I banged the side of the instrument panel and the L-Nav became normal. So, I took off and flew the flight without further problems.

Occasionally, the problem would recur....and banging it always cured it!
Never did anything more involved than that.

Maybe a bad internal ground? Loose circuit board? Who knows?

J4