Backfire
I would also check your exaust flange where it meets the head. Mine came loose and was doing exactly the same thing. (about a week ago, ran perfect before) Until it got really lose. While i was riding i considered that might be the cause. It got too loose and backfired everytime you let of the throttle. When i got back it was loose, so tightened it up and it stopped except once in awhile in 4th. And it still does it. Think the gasket blew out enough to not seal well enough. Exaust gas smells just right. And it is LOUD!!!! isent it!