The problem may not be in the distributor, but rather the induction system (my experience is to first check the accelerator pump shot prior to playing with the timing). What intake/carb or injection system are you using?

Once you have checked the induction, if that dosn't help, try changing the timing a little, if you retard it and it clears the problem, you need lighter weight and/or heavier springs if advancing it helps, heavier weights and/or lighter springs.

This is assuming you are running the correct initial and total timing to begin with.