My riding mate (he's got an 03 wr450) has just started having what we're hoping are clutch problems and not expensive gearbox problem. While in 5th gear under load the bike sounds/feels like the clutch is slipping. At first we though it might be jumping out of gear because of how suddenly it happens, but its not just free revving, its more of a slip. You back off and slowly accelerate again and you're ok - its just when you accelerate hard in 5th. The strange thing is it only happens in 5th. I would have thought if the clutch was slipping it would more likely happen in the lower gears where there's more torque. If you're just accelerating gently in 5th its fine. It's only when you accererate hard. Any clues?