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There are a couple of things that can cause this. First, I would check the motor. To do that, open the machine up and then apply 12-18v dc to the main motor. With constant voltage, it should run at a constant speed. If it does, then the motor is likely fine.
Follow the wires currently connected to the motor - that will lead back to the motor controller board. This board should be sending voltage to the motor based on how fast you want the motor to be going. If the voltages are not there, or they are not changing when you press the controls, then the motor controller board needs to be replaced. This is a very common problem.
If that's the case, let me know and I will help you to locate a replacement board.