Chrysler FWD auto tranny question

[The Warden]

Hello! My dad has a '92 Chrysler LeBaron 4 door sedan with a 3.0l V-6 and automatic (I think it's a 4 speed, but I don't remember; it may be a 3sp). The car's always (at least for as long as he's had it; I think it had 40K miles when he got it) had a hard shift between 1st and 2nd. More importantly, the tranny's been acting very weird as of late. Sometimes it'll drive normally; other times the engine'll rev but the car won't move very fast; almost like there's very little fluid in the torque converter (the fluid always checks out okay, though).

Any idea what's wrong? My dad thinks the tranny needs to be rebuilt or replaced; hence the car's actually been sitting for a long time (it's in otherwise good condition, though; I think it has 50 or 60K miles on it now). I don't know a thing about automatic trannies, so this would be a shop job...hoping for an inexpensive fix, but longevity (a relative term since this isn't a Benz we're talking about ) is more important.

Thanks!