I have seen that kind of behavior due to bad ignition coil also. I also saw it when a set of points had its rubbing block worn down so much that the points were almost closed. I am not one for replacing older technology just because the newer technology is supposed to be so much better. I kind of like the points and condenser primary ignition system design from back then. But, the other day a fellow enthusiast asked me if I could help him learn how to install a Pertronix Ignitor 1 and Pertronix Flamethrower ignition coil in his Mustang. Not one inclined to ignore someone asking for help with a vintage Mustang, I said I would do it. But, that meant I needed to get a kit like he was going to get, install it, and produce a video showing the entire process - plus explaining what the Pertronix does better than the old points and condenser design.
Well, last year I was dinking around with my ancient Sun oscilloscope, and found the dwell angle for our distributor points was at 40 degrees (at 45 degrees the points are not opening). That was a bit too much on the 73 Mustang Convertible (specs are 26-31 degrees of dwell). So, I opened up the point gap enough to wind up at 30 degrees of dwell. I captured that on video, and ended up posting it on YouTube at:
https://www.youtube.com/watch?v=vJvJ61MZagc
I did drive the vert a little bit last Car Season, maybe 150 miles or so. And as part of preparing for the installation for the Pertronix Ignitor I decided to get another dwell angle reading, just because I like to play with our old oscilloscope. I was surprised to see I was already at about 42 degrees of dwell, which mean the points are about to close (again), despite me adjusting them just a little while before. For that to have happened with that few miles driven since my prior adjustment is indicative of the distributor shaft lobes for the points opening and closing were not lubricated well enough, and the rubbing block on the points were closing up, again. Well, that made the installation of the Pertronix Ignitor well worth doing for myself as I decided I really do not want to be tooling around some day, and have the ignition fail because trhe points closed up (again) because of a excessively worn rubbing block., I share this because with an electronic ignition system, there will never again need to be any points/condenser replacement, or dwell angle adjustment, Everything is managed through magnetic field control of the primary ignition system with solid state electronics timing the saturation dwell angle (time) of the electromagnetic flux field of the ignition coil building and collapsing. When I tinker with the ignition with our old scope next year, in theory my dwell angle (points gap) will no longer be an issue. I never felt I needed the Pertronix Ignitor to correct something "wrong" with the old ignition system, until I saw how quickly the dwell angle I set the other year had increased almost to the point of become non-functional.
If you find the problem is because the ignition points have indeed closed up so badly, I would be unkind were I to not suggest you get a Pertronix Ignitor just to prevent an issue with points down the road. If you suspect the coil may be failing (rare, but it does happen) you can replace it with an oem coil and it will work just fine with the Pertronix Igniter. But, if you opt to get a Pertronix Flamethrower Ignition Coil please be aware the one you want has 1.5 ohms of internal resistance, and is designed to run on full battery voltage, not reduced voltage using the stock resistor wire for the coil's positive terminal on the coil. The instructions to install the Ignitor and Flamethrower can be a bit bewildering, but I finally got a good grip on their suggestions, I did a video of the Ignitor and coil installation and posted it on YouTube. In its description I provide links where you can access to my Google drive where you will find a PDF file with the Pertronix instructions plus my notesI felt folks considering a Pertronix Ignitor installation would find helpful.
https://youtu.be/_qGVhoFnJ50
There are other things, of course, that may cause the behavior you reported. If you find the ignition system is not the problem there is a myriad of other things that cause symptoms like that. I, for one, would love to see you post an updates letting us know if it was a bad coil or points about to close up (or some other repair), and perhaps more info with the updates will help also.
