340 utility gas timing drift

[John Cawyer]

First off... Merry Christmas all!

Now to business.
'58 340U gas that i've been chasing gremlins out of for a year now. I finally have it running smoothly... sort of. new plugs, wires, cap and rotor. pertronix ignition. Freshly rebuilt carb. tank cleaned. all new filters.
Periodically the timing will jump. Typically under a load it starts with hard miss and no power, then barely able to carry its own weight. fast forward 1 year of chasing around problems and I've figured out that the distributor is then off by about an 1/8th of a turn. loosen and turn CCW that 1/8th of a turn and dial it in and walla - it runs smooth as a kitten again. I'm now a half turn CCW from where I started after having done this repeatedly.
I cant imagine how this would be the pertronix. seems like a gear must be shifting in the dist or in the governor. anybody run into this before?
I dunno if it's relative, but I've also noted an occasional missed spark on 1st and 3rd plugs - indifferent to plugs/gap, wires, rotor, cap which have all been swapped around in troubleshooting. 2nd and 4th spark everytime. the other two drop a spark here and there.
I'm open to suggestions. my next step will be to try to identify #1 TDC and pull the dist and start inspecting.
 

[the tractor vet]

I would look at the centrifcial advance and make sure that the advance is working correctly as the cam may be sticking and not going back to 0 degrees or not advancing . Check the springs and flywights make sure that there is free movment in the points cam.
 

[Janicholson]

There are small gears in the distributor drive that could be missing teeth. The gears are located between the distributor and the drive housing (hooked to the governor) if teeth are missing that is the issue.
Less likely is teeth off of the governor to cam gear Jim