Transmission Jumps out of Gear

I have a 1990 Mercury Sable with 129k miles and while driving my vehicle at hi-way speeds it came out of gear and would not engage after letting my vehicle set for an hour I started it back up and went right into gear I started moving and it jumps out of gear. If you can get it to maintain in gear it will not shift out of first. Also you can start the vehicle put it in gear while holding down the brake and you can feel it come out of gear when it happened I managed to get it to a red light in a small town and when I braked it came out of gear. Need Help I replaced the nuetral saftey switch.