The reverse gear on some models, can wear quite badly in the "fork recieving" bit and also the shaft the idler sits on can be badly worn. my box pops out of 2nd on overrun, i know its the synchro, as i had the box apart to replace the layshaft, and in a massive rush refitted the old synchro again.... leaving the new one on the bench. (only realising after test drive) i havent had time to take it apart again since.
what i might do is rebuild another box that i have and ensure its all perfect, and then fit it when ready.
being as my engines just gone poof (possibly the head, but knowing my luck its the bottom end its running on all 4 but making a very bad chuff/knock/tick noise which doesent sound like its coming from the head and shakes like its running on one cylinder ) ... ill probably pull them both out,
i have wanted to do a "front end rebuild" anyway for a while.