The control module on down could be the problem. A symptom of a bad power stage(resistor) is as you describe or it may just cut back to a much reduced flow. There is a constant power supply to the fan motor and the power stage (resistor) controls the resistance in the path to ground. Certainly check the connection to the fan motor. If you decide to try replacing the power stage, it is located about six inches left of the fan motor. I think there is one screw and then it pulls out of the air box. There is a heat sink about three inches long attached and I don't remember just now if the knee bolster has to come out or if just the glove box.