i had a similar symptom of hesitation on initial acceleration from a cruising speed long ago. once i hooked up the ported vac advance, it responded to minute throttle increases much better, though i'm actually not sure why. so i suggest you test the vacuum diaphragms on the bottom of the distributor: put a clean hose on each one and suck. if you can suck air through, the diaphragm is broken and needs to be disconnected and plugged until you can replace it.
if you have two ports on the upper diaphragm, you must plug one port while you test the other because they share the same diaphragm.