Hello, I have a 1998 Ford Contour with 170,000 miles on it. While driving it on the freeway, it accelerates, on it's own, drops to another gear and moves back into the lower gear. It makes a big bang noise as it does this. i have had it to the mechanic twice now. They have rebuilt the transmission, but the problem remains. They thought it was a pressure valve in the transmission, but still has made a change. Any suggestions? Thanks, John Bills

Optional Information:
1998 Ford Contour 4 cylinder

Already Tried:
rebuilt the tranmission.