Hello. My name is Michael. I can help you with your question if you still need assistance.
The first thing to test is the the motor itself - apply 12-18 volts DC to the motor. If it runs at a constant speed, then it is likely ok. That is what I would generally expect to be the case.
More likely the problem is with the motor controller board. When you adjust the speed, the console sends a signal to the motor controller board, which then varies the voltage to the motor. So, please test the voltage coming from the board to the motor when you are changing the speed. I suspect you will find that the voltage does not change when you change it from the console. If this is the case, then the motor controller board needs to be replaced. This is the most common failure on this model.
I don't have a reference to that model number, but I do have a PFTL78807 which is a Proform machine. The replacement board is.
Here is the replacement board for that model: