Clutching at broom handles!
Sounds a lot like the clutch is dragging. As suggested, check the cable routing, check to see that the cable outer has not been crushed so that it is gripping the inner, and check adjustment (should be a few mm free movement of inner cable at the lever). If these are OK, then it may be a warped (slightly bent) metal plate, or the fingers of the clutch basket have grooves in them, thus not allowing the plates to properly separate. Do you have any idea how many kms old the clutch is?
I don't know about the XT600 but on other Yamahas I have held the damper rod on the inside with the end of a wooden broom handle. It needs to be a thin broom handle, and you have to hammer it in a bit for it to bite into the socket in the head of the damper rod. It's an easy thing to try. Also Flyingdoctor is right, an air wrench usually does it, but I have had to use the broom handle AND air wrench on several occasions.
Nigel in NZ
The mouth of a perfectly contented man is filled with .
-- 2200 BC Egyptian inscription