1999 Ford Windstar transmission

My wife had driven about 70 miles when she stopped at a stop sign and proceeded to take off the transmission slipped. Instead of stoping she coninued on ove about 1.5 miles of steep hills. When she started up the next hill it would'nt even pull the hill or go into reverse. She got out and smoke was boiling out from underneath it.I let it sit a few days then refilled the transmission, almost completely empty. It has run fine no slipage for about 3 months. Never determined the cause just prayed about it. Now when you get going for awhile at a good pace then slow down to make a turn and accelerate again you can feel it shift hard?Whats the problem?