Why does my OD Off light starts flashing at higher speed for extended time?

The light comes on when I turn off my overdrive normally. When I travel at 80mph or 120km/h for about a half an hour (sometimes it turns on sooner), the light starts flashing and continues to flash until I shut the car off and restart it, or just drive slow for a while. The car is a 1996 Mercury Mystique LS.