For nose-wheel shimmy, servicing the shimmy dampner is generally only fixing the symptom, not the problem. I'd try the following:
1) check all the steering components are tight. They loosen up and can end up with a good deal of shimmy inducing play.
2) Replace the tire, paying special attention to the red dot on the tire and line on the tube, followed by a spin balancing (if you don't have the machine, a local motorcycle place might be able to help with this).
-me