Transmission Fluid Leaking problems of the 1997 Ford Aerostar

My transmission developed a bad leak in the front seal. I paid to have the transaxle removed and the seal replaced. The fluid was changed as well as the filter and then shortly afterwards the cars overdrive stopped working. It will shift into overdrive but it will shift right back out when I step on the gas even the least little bit. I could not afford to have it fixed so I put it in drive and drove it for about 3 weeks and yesterday I was doing about 60 and the car shifted into second gear for no reason and then right back into thrid. Then about a half mile later it began shifting from third to second and back over and over. Talk about getting jerked around! it was like someone stepping on the brakes! now I am on foot and I am on foot and due to our great economy I am out of work for the time being. They should offer a manual transmission on all their cars.