You might want to check the shifter cables, they maybe lose or binding. The other thing that I can think of is to check the detent's, make sure they are tightend all the way down, if they arn't that would explain why it can pop out of gear. If you aren't fimilar with the tranny you can see the detents in the second picture down here:
http://www.twingles.com/ideck/rebuild3/dretranny.htm They are the three bolts in a row just below the revers light switch.
