I have a T10 transmission in my 65 and it pops out of 2nd gear occasionally while driving. Could this be a worn shifter fork issue or might the transmission need to be rebuilt?

 

Not related to shifter fork. Problem is likely worn or damaged engagement teeth on the 2nd. gear blocker ring. Possible damage on 2nd. gear synchro teeth, but much less likely. Also possible broken or slipped out of place synchro ring inserts. Fix requires disassembly.