Need HELP! 2003 Town Car, 4.6L engine, 4R70W transmission, the problem is a stuttering feeling in high gear, between 35-50 mph under a light load in 4th gear. Stuttering is not felt on a level road, under hard acceleration or in gears 1-3. Transmission torque converter has been replaced, per dealer recommendation. Problem still exists. What could be the cause?