Transmission problem

I have a 1996 ford windstar. Mileages is 118000. According to the dealer, a new transmission was put in at 80,000 miles. It has been shifting really hard into 2nd gear. But only when I hit the accelerator hard. Yesterday, I was driving down the interstate and the overdrive light was flashing. The RPMs were at 5000. It was really hard to get it up the exit ramp trying to keep it going. The fluid was all but empty. We put 2 quarts in and went another 30 miles. The overdrive light started flashing again and the shifting started messing up with the RPMs. We pulled over and found a major leak of tranny fluid. We had about 150 miles to get home. So we kept putting a quart of fluid in every 40 miles. Today, we cant find a leak and the fluid says full. But it still is messing up trying to shift. It revs up when the throttle isn't pushed and gears feel as though they are slipping. What could be the problem? And how do we fix it? Thanks.