Transmission Falls out of gear or something

I have a 92 Lumina 3.1 and it runs greats rides smooth but when I stop it seems to fall out of gear into neutral. If I step on the gas it will eventually catch and go but generally not before atleast one person honks. If I manually put it in neutral once stopped in back into gear there is no problems. I have replaced the filter/gaskets and cleaned the pan Any ideas ?

If you never serviced the tranny before and this was your first time, you just got yourself a problem.