Vehicle stalls when put into gear

My 91 firebird formula has been sitting for 7 months while I was deployed.. When I came home my battery was dead so I charged the battery and the car fired right up with out a problem.. I drove it down the street a few times and on the last trip down the street she stalled out on me.. Now when I start the car and let it warm up and go to shift it into gear she stalls right out in reverse or drive... I checked plugs wires did oil change.. Transmission needed fluid I did that.. I replaced the O2 sensor because it was giving me a code and I replaced it and it was really burnt out.. there are no more codes and I disconnected the battery and reconnected it and let it run for about 3 hours to get my car to relearn itself but still stalls out once the vehicle is warmed up going into gear.