The car was driving great except a hesitation when accelerating from a stop. We had been told that the throttle positioning sensor was bad so we had that replaced. Now the car will not go over 25mph. Mechanics claimed the exhaust was clogged and needed replaced. (we knew it needed replacing but only due to some rust). We had it replaced at our muffler shop and it made no difference at all. Any suggestions as to what to look for?