Help plz, 97 ford contour!?

the 4 spd trans Goes into overdrive but, downshifts on expressway after a while of driveing. How can you tell if the transmission range sensor is bad?She does fine in city driveing. The o/d off light begins to flash when on the expressway, about 25-45mins. As that occors, i can;t get it back in to overdrive and starts shifting hard afterward. If it Sits for a while then it will go back to normal and shift fine then the issue returns again. Anyone have this issue? Missing a muffler, can that cause this?