Have you adjusted the rectangular shaped block at the back of the engine area. That block is what keeps it in gear, usually. It has one bolt in it, loosen it, let it move a bit, then tighten it up, and try it. It worked for me!
Your model, if a late model, may not have that adjustment! Getting into the transmission, with no knowlege of it, will surely bring more trouble than you need! They are NOT like the average transmission, as we know them. If tranny is bad, better seek professional help!

I realize this is an old thread, but someone might run into it browsing, as I did. If everything stops when it pops out, I can't help you. But if it's the wheels that stop turning while the rest still goes, you need to check where the rod that works the wheel clutch goes into the transmission. There should be a collar with a bolt down there. Tighten that up until the rod no longer turns freely. At least it worked on my old ('82 I believe) Pony. The new ones are a bit different.