Hello. My name is Michael. I can help you with your question if you still need assistance.
It sounds like the motor is working fine. But the problem sounds like it may be with the motor controller board. The symptoms are so indicitive of this, that you may not even have to check the voltages, but you can if you want to verify. Check the voltages coming into the main motor. At a constant setting, the voltages should remain constant. If they do not, that is because the motor controller board is sending the wrong voltages and causing the speed to increase. The solution is to replace the motor controller board, which is a simple replacement. It's a very common problem and the part is easy to get. Here is a link to it.
Please let me know if you still have questions.