I'd mic the clutch fibers, see if there out of spec. If there in spec probably the springs. I've tore k5's down thinking the fibers were wore out. They werent, it was just the springs. Those springs get heated and cooled alot and finally loose there tension. I personnally use heavy duty clutch springs on all my bikes, I hillclimb quite a bit with over size tires. Slip the clutch quite a bit going up the hills. The clutch lever is also harder to pull.
