Jeff, in post 36 you stated you tested the coils, but didn't specify how, are you talking about a simple static resistance test on the two coils? I'm not sure that's going to be conclusive enough to rule out a dynamic issue. Before you were claiming that the problem only occurs when coasting with the throttle closed @4250, but accelerating @4250 its not present. Your latest comment about Hal modulating the RPM in the garage with you feeling the exhaust suggests its 4250 RPM dependent and not necessarily throttle+RPM dependent. Did I hear you correctly? That sounds like it could be a possible coil issue, I'd swap the coils/wires with Hal's bike and see what that tells you, it will at least rule those two variables out if nothing else. If the problem goes away, then swap either wires or coils back to see if either or both are the cause. In fact, I'd pull all the coil wires out of the coils and spark plug boots and inspect them on both ends, you could have corrosion in there that's contributing to the problem.