You have a bent shift fork and depending on how long it has been doing it you will need 2nd and 6th driven gears and new shift forks. I would get the tranny backcut while its out and put in the billet shift shafts. You can change the forks with the motor in the bike if you are experienced enough. You will have to pull the exhaust, oil pan, complete clutch ass, sprocket cover, water pump, and gear shift cover to change them out.
SAE Performance (site sponsor) has the shift shafts.
I have had great luck, prices, and turn around time with MRE on the tranny work.
RonAyers.com for the forks