is the autronic a piggyback or full control stand alone?
if its piggyback then the revlimiter or speed limiting will cut it at the higher speed in the higher gears.
probably it is under more load at those revs in the higher gears and some link in the electrical or ignition system breaks down under the heavy load, its then just a process of elimination, plugs, gaps, heavier leads, coils u have already done, all electrical contacts and so on.
but yeh, another dyno run expense on top of the other parts expenses sucks.