I say it's some sort of lean condition and I'm an expert since I once went through a Cat 3406B injection pump and I rebuilt a hemorrhoid valve on an old mechanical Mack. The next guy might suspect the muffler bearings since his 30-something year old cousin that looks like the Real Slim Shady once had a problem with his muffler bearings.

I'm with Fender Bender...Call Don! He is a real nice guy. He knows his stuff. AND he knows the bike, or atleast it's engine.