I've noticed that too... My MTR's would ride just perfect up to about 80mph before I start noticing a bit of wobble. After I rotate them, vibrations would develop, so I've given up on rotating them. They seem to be wearing just fine by themselves <img src="/forums/images/graemlins/kewl.gif" alt="" />

I think it's due to the lugs wearing a certain way - if you look closely at your lugs, you'll see that the leading edges of the lugs are more worn than the trailing edges. So when you rotate the tires, it forces the lugs to wear in the opposite direction. If this is the case, then the best way to rotate them would be front to back, without swapping them sideways. Give it a try...