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Falling out of gear at low speed sounds like fluid pressure starvation, but when speed is increased, pressure builds up and overcomes whatever is blocking it at low speed.
How many actual miles are on the car? It may be overdue for a rebuild. One of the worst things about these Ford-O-Matics is old dried out and worn rubber gaskets and O-rings, especially the front converter O-ring and the converter hub O-ring. If the trans sat for long periods of time, you know they had dried out, but they may have cracked which means there will be little annoying leaks all the time.
Years ago I had a lot of trouble finding a reputable and professional shop whom I could trust with my Ford-O-Matic rebuild. I passed up three places in different towns whom I felt un-easy with after talking to them in person. Finally found a shop in a different town that is very good, trustworthy and does quality work. Eventually, if you search, you can find somebody.