[DW31S]

That should be an easy thing to diagnose and yet, will make you pull out your hair. How about fuel volume and pressure? Fuel filter? Do you have access to a known good distributor? Have you checked all grounds? Engine ground to chassis? One cylinder not firing exactly perfect would cause the eradic idle, but engine should still rev over 4500. Gotta get back to basics and try one change at a time. Dwell 28-32 (or pop in electronic to rule out dist.) timing all in by 3000rpm and 36 degrees for street should be good. Fuel pressure around 7 should be good. Could be a minute crack/cracks in cap and cross firing inside and invisible to the naked eye. Dead valve springs will cause same symptoms, as will valves out of adjustment.