How old is the distributor, my guess is bad reluctor/ reluctor gap . Is the distributor the mallory type or is it older? Is it a mopar performance distributor or the new crap you can but at oreily/ autozone now- they are easier to work on but not well built imo. What advance springs are your running. What is your timing at 3,000 rpm and at 3500 rpm? if you set the initial at 20 you could be well past 35 mechanical advance at 3,000 rpm- that could cause a problem- Are you sure the vacuum advance is working does not have a damaged diaphragm. Put a mity vac on the vacuum pod and see if the distributor plate moves when you apply vacuum. What coil do you have, I like to run the msd blaster (2)? coil - cant remember i always remove the deals and paint them black