More than likely it's the control circuitry or speed sensing circuitry. It might just need cleaning a photodiode somewhere. I don't think it's a software problem.
You might write to the manuf for troubleshooting advice. They might sell you a "factory service manual" but first find out what kind of test equipment is necessary.
For troubleshooting to the circuit board component level, at a minimum a schematic is necessary along with recommended adjustment procedures.
The most likely cause for the motor to speed up and slow down is a dry or worn
walking belt or walking board. Lift the walking belt and feel underneath for
worn spots on the board. If there are no worn spots you can lubricate the belt
and board with belt lubricant from Sears. It will have an
instruction sheet to tell you how to apply it.
You could consider putting the console through a calibration mode. Without a lot
of technical expertise, there are risks involved. I would not really recommend
you try this as electrical shock and damage to the controller are possible.
These instructions tell you how to adjust the minimum speed by adjusting a
potentiometer on the motor controller board, and must be obtained from ProForm.
All Parts (including the recommended lubricant) can be found
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