I have a 1998 ford Fairmont Automatic and recently got it back from the mechanics but have noticed it starts to drop in revs when in gear, and tries to stall. it's fine in park though.
Every automatic drops in rev´s a little, when in "drive". The torque-converter adds some load to the engine.
It should not stall though - if it does, there is either something wrong with your motors idling state or your transmission has a problem which results in an unusually high load.
To prevent further damage, make sure to check transmission fluid level before turning your car on again!
It´s hard to give more precise tip without you giving any more info. If you just got it back from the mechanics, they should be able to advise you ...