I agree with Jerry Ross, at the same time you could have one weak wire or a bad cap. Once you take off the motor is no longer under a load so any intermittent misfire will come out of nowhere. Cap, plugs wires. Or a bad engine ground, I would start at the cap and clean it or replace it if it is badly burned. Wires ground them out one at a time if you have a bad the spark will ground out.

Sounds as if you need to be sure plugs are not fowling out and be sure fire from dist and coil is ok.