I have a '97 XR600R that jumps in & out of gear under throttle when it's in 3rd gear. When you're downshifting it's fine, it'll stay in gear. But, wick the throttle and it jumps in & out of gear. No grinding that I can hear or feel. This doesn't happen in any other gear. I checked both a factory Honda Service and Haynes Manual's troubleshooting sections and couldn't find any diagnosis. Any ideas on what could be wrong with this patient?