they geared the bike taller in 06 on,,so at highway speeds it didnt vibrate as much,,but the fjr really vibrates between 6500 and 8500,,i can feel it in my *** coming through the seat from the frame,,,n my friends 08 does the same thing at the same rpms,,,i have 65k,,it is just where it happens to vibrate most,,they r balanced to be smoother at certain rpms,,,it was something i learned to live with,,,did u ever wash your bike??,,n think it ran better,,lol,,lol,,i dont think changing plugs will change the balance of your motor,,i changed plugs at 30k and at 60k,,they looked just like new,,gap was within tolerance,, they were light brown n clean as a whistle