It makes me wonder about that comment you made about a timing jump.
Do you know how to verify that?
(The distributor rotor won't be pointing at the number one plug terminal on the distributor at what the timing marks say are TDC.)
I would say it could be off by as much as 1/4 inch, and if you were to try and compensate for that with the distributor position it just runs worse, or not at all.
Try this it works pretty well.
Use a marker and make a mark on the distributor base and block (like a line from one to the other) to mark the position.
Put the damper and timing marks lined up at what your initial timing is (zero line on the damper at say 10-12 BTDC mark) or whatever your initial is.
Pull the distributor cap and use the marker to make a line down the side of the distributor about an inch long from where the cap sits on the rim downward right under the rotor tip.
Now put your distributor cap back on (just set it on if you want)
Is your number one plug wire directly inline with the line on the side of the distributor?
If not something is off.
Also it's possible you have a rotor phasing issue (meaning the rotor tip is not close enough to the right terminals inside the cap when the distributor fires the coil)
While you have the distributor cap loose and the damper set at your initial timing number look close to see if one of the rotor vanes is directly lined up with the little metal bar in the center of the pickup coil in the distributor.
These simple methods will tell you if you have a jumped timing chain or a rotor phasing problem.
Preakish made me think of one other random possibility that people fight for days on end.
Check for continuity between the two ends of the coil wire and make sure the core isn't burned out of it.
A spark could jump the gap caused by a burned out core until the engine load increases from being put in gear and then it might not be able to jump that burned out gap any longer.
(The more pressure in a cylinder, the harder it is for the spark to jump a gap)
Sometimes the gap inside a coil wire gets so long all the engine will do is idle in neutral. (Seen it multiple times)
Also seen rotors with a hole burned down through the center cause the same problem.
Cylinder pressure increases and causes the spark to go down through the rotor to the top of the distributor shaft.
This is a lot more common of HEI or other high powered ignitions, but it happens.
