[XP6]

I had one that did the same thing but wouldnt rev past 3500rpm, after about 4 mechanics I took it too an old school(RIP Harry)fella within about 5 minutes he diagnosed the dissy shaft had to much side ways play causing the points dwell angle to jump all over the place.
New dissy fixed it, so find some who has a dwell meter and see If the needle swings about wildly.