My car does this as well, though getting your transmission rebuilt is probably not the answer, as mine was rebuilt just last year and is still under warranty. When I am on the highway, I drive for about 15 minutes, and the OD light strts flashing. This does not happen when I am driving around town. When I pull off of the highway, my car shifts really hard for a while, eventually going back to normal. I have never encountered anyting like this, but it's good to know that I'm not the only one with such an odd problem.

1996 Contour GL AT, 2.5 V6 No mods, rollin on 14's with hubcaps, 116,000 miles, and still havent been beaten by a ricer.