A worn clutch basket/hub will also make nuetral had to find, but I agree w TMKIWI that it may be the shift cam stopper or worn shift fork.
At the age your bike is, you should probably split the cases and overhaul the lowerend and tranny. new bearings, seals, check for wear on the gears, and shift forks, and replace anything that has wear. I believe that Microblue has a ceramic bearing kit for your bike, I hear that they are the bomb diggity, as my kids say. And the reduced friction increases the horsepower signifigantly.