Varying spark, out of ideas

Any backlash in the drive gear? Pull the distributor and use a large screw driver the twist the gear back and forth to see if it has excessive play. don't get carried away twisting the gear, or, it might pop out and move a cog or two. Just take up the lash if it has any. You can start by just rotating the rotor back and forth just taking up the lash too. There will be some, but, there shouldn't be a lot.