I have never done this so please excuse my fumbling.        
I have a 93 Grand Cherokee w/auto trans. I just had it rebuilt because it wasn't shifting right and then started to go out of gear.
After I got it back it still goes out of gear after short drives. IE; Go to store and then when I leave it will be OK for a short distance and then if I stop for another vehicle it goes out of gear when I try to move again. I have found twice that if I shut it off and restart it it's fine.  
My mechanic is stumped because he can't get it to do it.