i have a 1981 fj-40, i just changed the factory carb and went to a weber 32/36, and went through and replaced plugs,wires,dist cap,etc. i also went through and checked all the valve clearence and rockers. set # 1 piston tdc reset the distributor to set the timing. when i fire it up , on every(approx)4th revolution the timing will bottom out and almost quit them will level out for about 4 more revolutions. the only thing i can think of is
1) that little ignitor box on the coil or the coil itself.
2) i am a dumbass and i set it out 360 degrees( which i dont think so because i did it 4 or 5 times.

could the ignitor module be the problem or is it something else?

just a suggestion-i had a similar problem and it turned out to be my distributor, the stupid shaft had bent. if possible, get it on a scope and make sure that distributor is acting properly,
Cruiser-B