silly question but checked the distributor for wear and wobble?
on my old 5 i did this, exact same problem occoured.
what had happened was the dizzy had been worn (with age) and that last rev over what you are suppost to rev to must have popped something out and the dizzy didnt spin on regular intevals, putting the timing way retarded on some and wayy too earlly on other beats.

if not try the choke (if its automatic) see if thats staying open on starting, could also show the same symptoms