94 Ford Contour shifts real hard and slips in and out of gear.?

4 cyl automatic transmission starts out fine but then starts shifting in and out of gear. Sometimes it feels like it drags or is not engaging. Occasionally it slams in and out of gear. I was told by a mechanic friend of a friend sort of thing it might be some sort of pin that wasn't placed properly when the transmission was replaced. Any help is appreciated as it is the only car and recently got laid off.