I had the same thing happen. Loud squeeling noise! Cap and rotor were fine.
Removed the distributor and the bushing the shaft rides on had a varnish build up on it and it had resistance when I turned it by hand. I pulled the shaft, cleaned and lapped the bushing greased it up and it's been fine since the repair, 6 or 8 months now.
You have to be carefull if you lapp or hone the bushing, you dont want to take much material off or you will lose tolerance and the shaft will wobble. Then you have to replace the bushing or the distributor.
A good cleaning and lube of the bushing and shaft fixed mine.
Good luck.