have you checked the gap between the pick up coil and reluctor tips As I recall the gap s/be ~ .006-8" and checked with a brass feeler gauge. The two should be parallel to each other as well with no chipping of the tips.
One could also try another pickup coil.
Might also check for excess play in the shaft.
I have also in the past run a small braided ground strap from the control box to the engine to make sure the box, egine and distributor are grounded to each other.
Really stupid question, Your distributor clamp is tight, Correct?