I suspect the real problem is the timing gear is stripped. The gear is made out of a fiber type material and after years of use, or even long storage they sometimes break. The break happens around the core/center, where it attaches to the cam shaft. It is probably able to turn a little as the cam rotates, but it isn't moving as frequently as it should so it is sometimes just turning. but only enough to cause an occasional spark. This same thing happened to one of my dad's Model A's many years ago. We found out what was happening by checking the timing, using the removal screw/plug and found out the timing gear wasn't moving around as it should have been doing. Had to tear the engine apart and replace the timing gear. It ran perfectly after that. I hope this helps. Mike Turner