I had the head shake issue on mine and finally figured out it would go away if I loaded the rear or just sat on the back seat. Try that for the shake. My final answer was a stabilizer, and glad I did. As far as power agree it is pretty good in the 5-7.5k range. I've had it up past 100mph several times, it will do it although I find the motor likes about 80.
I would call it head shake. It calms down if I decelerate but it might be strong enough to induce a tank slapper if it happened at the wrong time.
You might be right about the rear pre-load. I am a big guy 255lbs. and I am usually packed for a long trip, add another 50lbs. I have the preload on the second to the stoutest setting and the screw about 1/3 rotation out from closed.
