Looking at a MC2100LT motor control board out of a Proform / Icron treadmill. The controller, from time to time , inexplicably , and dangerously surges or decreases the motor speed. Removed controller and motor from treadmill and placed on a workbench in my lab. Used a PWM controller ( 20Hz/18%.. ) to bypass the console input and have added a series resistor ( 480 ohms ) to the PWM input to the FD817 optocoupler to respect the data sheet. I have a 2 channel scope ( 222 )( ground isolated ) and DMM to check most of the likely places of reported issues. Motor controller comes up from soft start fine, runs at speed, varies with the change in PWM percentage input. After a few minutes it will suddenly increase the pulse width to the MOSfet driver with the same input PWM on the input ( pin 4 - HD2 ) . This also can occur right after powering up the board. Pretty much with the wealth of information provide on the MC2100 threads, I have found nothing out of order, Voltage regulators on the money, SCR motor pos voltage seems stable while running, so it comes down to what I believe to be the micro, which is outputting the PWM signal to the MOSfet driver. While there is some documentation on the reverse engineered schematic and MC2100/MC2100E motor control document posted, It is unclear to me what is affecting the micro to vary the PWM signal in this way. Has anyone seemed the same thing on a MC2100 ?