I didn't have an answer for you before, but maybe I do now...
It's a possibility that your rev limiter may be failing!
(It's the easiest to troubleshoot)
On the lower front of the engine, you'll see a 2"×3" box... that's the limiter,
there is a ground wire for it attached to the left mounting bolt,
remove the bolt and the wire eyelet from it, reinstall the bolt, and tape the eyelet up, and test drive!
If it still happens, then we'll move on to the coil!