Does it take throttle, and rev ok?
I had one case in which like yours the timing jumped, and I mean it was here and then there, with no transitions of in-betweeness. In my case, the engine did Not like to rev up, and the jumping strobe got worse and worse.
I found the problem in the reverse connected pick-up coil. IDK how that works exactly, but after I reversed the polarity, I had to reset the timing cuz it was quite different. After that is was normal.