I have a small problem with this bike. I can ride for about 15 to 20 minutes and then the rear cylinder stops firing. Sounds like i'm on a lawn mower. I've checked the ignition control modules, and the coils, both are good. I've replaced the spark plugs, and plug wires. I have cleaned and protected the battery terminals too. I'm at a loss and can't figure where to go next. Can any body help?