Not so sure it's ignition at all. 58 degrees dwell is close enough and 13.8 volts is perfect. New coil, strong spark, the only thing left to do is check that it advances and retards correctly. Now you're positive it isn't ignition. Nice that you checked because you always, always, always check ignition before you mess with the carb.

You also do a quick spot check of your valve lash, as a matter of course. This is your pre-carb ritual.

Now. Your symptoms sound stereotypically like debris in the carb. You're running lean, on and off, as debris partially clogs the jet from time to time. No big. Just take the carb(s) apart, spray everything out and reassemble, do your fuel filter, and as long as you're careful not to accidentally "adjust" anything you should be back up in no time. This is an exceedingly common problem on carbureted vehicles. An hour with a screwdriver and a can of carb spray is all you need.