273 that runs great, sometimes

Hello FABO members. I am a new member and a new owner of a 1965 Barracuda with a 273ci with a Carter 2bbl (for now) and 4 speed transmission. My current issue is that the engine can run very strong with no issues, or it can shake, and it appears to be dropping cylinders causing it to run extremely rough if you can keep it running. I have done quite a bit to hopefully correct the issue but I have had no success, just a lot of heart burn. Here is a list of what I have done so everyone will be up to speed as much as possible.
New plugs, points, cap and rotor. Sent the Carter 2bbl out for rebuild. New fuel tank, lines and filter. All new non-ethanol gas. Replaced the ballast resistor and new ignition switch.
Timing is set at 8 degrees btdc, the dwell is 34 degrees. Vacuum advance appears to be working as it should.
Unfortunately the issue is intermittent which makes it hard to pinpoint what is going on. Yesterday I took it out for a run, everything started out fine. About 1/2 mile down the road the motor started to miss, and it would get worse when I gave it gas. Turned around and before I traveled back the 1/2 mile home it was running strong again.
Any help you can offer is greatly appreciated.