After a little thought.....
If by chance you somehow lost valve clearance on one exhaust valve
it would pop when that cylinder fired.

Some of the symptoms you mention seem to be consistent with a fiber
timing gear loosening up on the steel hub.

You measur
the valve lift at the valve spring retainer from the top surface
of the head, take the difference of the height closed and open. Should be
around 3/8" or a little more.

Don't worry about advancing the ignition too far. It will CERTAINLY let
you know if you do, so you can correct it then. I routinely set mine to
20°or more (don't disconnect the vacuum for that setting)
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma