Car: 1996 Ford Contour GL..2L motor...4speed automatic..with overdrive button.

Driving car on highway...see the overdrive (green light on dash) flashing..on and off. Pull of the highway. Start to take off...and notice..as car shifts between 1st and 2nd..
a "thump feeling" and noise. Later in the day...in town driving...this disappears. Thump and flashing o/d light.
So...basically...anytime I drive the car on the highway for 15 or 20 more minutes..I have this same problem..o/d light flashing and then problem (thump, noise) between 1 and 2. But if keep car in town...lower speeds...problem disappears and does not come back..unless I go out on highway. Any thoughts are greatly appreciated. ( for the record..I have been told torque converter bad...or valve assembly..or sensor..on and on.) No straight answers.