'99 speedometer falls, engine revs

this is my cousins car, and I've never been around when this happen, but when she is driving the vehicle the speedometer falls, and then the engine revs up, she said it's like when you have a bad clutch, or your tranny is low on fluid. It's obviosly something computer controlled, but we checked the tranny fluid anyway. They took it in, and replaced the speed sensor. Thats what the computer codes told them to do, which makes sense because the tranny is controlled by the speed of the vehicle, which would cause it to down shift, and the engine to rev. It seemed to work okay for a short time, and then started doing it again. It is possible that they got a faulty sensor, but I was curios if any one had seen this before, and could help me out

bad Vehicle Speed Sensor