If you are running an early shifter, they can wear out. There is a set of shims and bushings in the shifter mechanism itself. You may have to work the shifter real hard to get them back in alignment. If this turns out to be the case, then its time to have the shifter rebuilt or get a Hurst Competition Plus.
Also check your shift rods and make sure the bushings are not worn out. They came with nylon, but steel were available.
I can't imagine there is something wrong internally, but you never know. Check these things first and let us know.
