Is it caused by abusing the clutch?
Yknow... lots of revs, riding it too long, burnouts (only in the wet with 45hp...

), getting stuck in 3rd, far too many cross city stop-go sprints...
*guilty as charged*
Found that the slipping currently in mine can usually be brought on by three or four quick takeoffs (or, for testing purposes only, one burnout*) after which it will slip like buggery in 3rd and 4th for a bit, before some kind of self adjustment (or maybe just the parts cooling off) kicks in and it actually ends up more crunch-happy than before.
Hmmm.
* which also confirmed my thoughts on the grippiness of each tyre. Locked rear wheels failed to hold it still against 'better' fronts, and the stickier front left pushed it sidewards against the less tready right.