its the snap ring on that gear thats causing it to jump out (snap ring is located right on the syncro ring for this convos sake).... the snap ring gets worn with time, and they are what holds it in gear, so when they get worn out they can no longer do there job.... if the clutching teeth are worn out on the gear they could also cause this prob but this would cause a grinindg sound if it happend....

hope that helps
heres a link for what you need, has the snap rings and the syncro rings
http://shop.synchrotech-transmissions.c ... cstrfrnt03