Just bought this bike. 6100 miles. starts fine, idles fine, runs real well around town (under 45 mph). Plenty of torque at bottom end. Occasional backfiring when off throttle and coasting in gear to a stop. Get up to highway speeds (55-70) and it starts sputtering like it is running on watery gas. I have filled it with 93 octane twice and added gas line dryer. I am going to check plugs next. Any other thoughts?