360 Running Issue(s)?

I was thinking the same thing about charging voltage while its running at the point when the problem occurs. An over-charging electrical system will have adverse affects on the ignition coil saturation & the ecu. Same thing if its under-charging. Also eliminating the tach from the equation would be good as well as you may have a tach shorting out or a connection problem there. You didn't mention if the dist cap & rotor were replaced, I'll assume so since it's a new dist.
I'd also suggest if the ignition checks out taking a good look at the accelerator pump in the carb. Good luck!