[Barnsnake]

As I remember, you have your centrifugal advance starting below your idle speed. With that arrangement the motor will be challenged to find a consistent idle speed. Any little thing that drops the speed lower than normal will retard the timing and keep the motor from coming back to "normal". You may be spot-on with the temporary mixture issues and the timing retard prevents the engine from recovering on its own.

You might be able to reproduce the issue with a timing light attached. Do something to make the engine stumble (maybe choke the engine a bit with a rag or the choke butterfly) and see if the timing stays low and the engine fails to recover on its own. If so, blip the throttle and see if the timing is more advanced when the engine returns to idle.

Keep us posted.