My ribcase did that as well. The cause was that the fork would slide the outer gear too far in the 1st direction to the point where the balls and springs would come out just enough to lock it in 1st.
Now I'm doing this from memory, but the solution is to add a washer on the rod to limit travel so this can't happen. It just floats on the rod and hasn't caused any problems.
I owe credit for this to the guy who sold me my current box. He had it happen to his daily driver after work one day. Just imagine driving home from work in Boston in 1st gear; I don't think he made any friends that day. Now he does it to his street car and his racer.
I hope this helps.