Did you or someone else recently services your clutches?
If you did, it's possible that you didn't put it back together properly. There are usually marks (I think an "X") stamped into each sheave that should line up with each other. This is the position that the clutch was balanced at the factory. If you didn't line up the "X"es, it may be out of balance as a result.
It may be possible that extreme heat has caused the sheave to warp as well. You may very well need a new secondary then.