One test to try is put a volt meter across the motor wires (as close to the motor as possible). Then see if the voltage drops when the speed drops. If so, that is telling you something upstream is limiting the current (and voltage) to the motor. If not, it is something inside the motor that is messing up. On mine, it was worn brushes in the motor causing it to overheat and slow down after running for a while:
- http://www.4crawler.com/4x4/CheapTricks/Blower.shtml#MotorRepair

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