This was a problem also with the old GM distributors that the MSD advance system is patterned off. The weights or the shafts would bind up--solution was to take it apart clean shafts and all weights, pivot pins lube, etc
Another area to check is on engines that the dist goes thru the manifold, if the block has been decked or heads milled the distributor gear is loaded against a boss in the block causing the upper end to be jammed!! Also a
oil pump drive shaft that is too long can do the same. An assortment of aftermarket parts exposes to a tolerance stackup that gives results out of the norm, but none of us use any aftermarket parts in these cars, correct???
Jerry