mystery problem in 92 cougar

my cougar had been sitting for about 2 months because it needed an alternator, and after changing it out it started fine but when i put it in gear (any gear) it would only go about 5 mph w/ the accelerator completely floored. it eventually died out and after letting it sit a sec and then it started i let it warm up and it stayed on but the lack of power to the tranny persisted. any ideas???