Try cleaning (soaking in acetone) and relubing the one way bearing in the tranny.. I went through the same deal with the 2speed on my S30.. Never did figure out why it would only run in 2nd when the arm on the clutch was not locked open, and even when I had the set screw LOCKED down tryin to make it stay in 1st, it'd still just run in 2nd.. I went to the single speed tranny and never looked back, but  after much thought, I think the only other thing it can be is the one way bearing.. Good luck!