My sister had a late 90's Lincoln Town Car. When in overdrive, it would shift in an out of overdrive, or so it seemed. Long story short, it was the torque converter not locking up properly. The fix was a new torque converter. Sometimes it felt like you were driving across a long cattle gap. Hold the brake pedal just enough to activate the brake lights. If the brake lights make it stop, it is the torque converter. The torque converter does not lock up when the brake lights are on. This helps the car slow down using engine braking. Bill