Treadmill DC PWM motor Controller

My treadmill motor speed is surging at times to the point where the lights in the room dim. I have attached a schematic of the controller I made up and a picture of the controller itself. It has a PWM motor controller that receives a signal voltage from upper console on pins #2 and #3 of connector A1. Voltage varies from 0.8v to 5.9v according to requested console speed. Also voltage does not change on pis #2 and #3 when motor surges. Am I correct to say the DC motor speed is controlled on the negative side of the motor through pin #7 of the mother board? I checked the diode rectifier bridge with multimeter in diode setting. Seems to check out ok. Any help diagnosing this problem would be greatly appreciated.

If the lights are dimming, the motors are trying to turn under a BIG load. The question is not likely the controller but, instead, the unlubricated and or depolymerizing/sticky tread that the poor motor is trying to pull across the skid plate. Or a very dirty/sticky skid plate. It is shuddering / stuttering because the stickiness may occur in a narrow temp range as the sugar, plastic or whatever melts and hardens.

A tube of silicone oil should have come with your treadmill. Use it after you clean any crap.

IF, you were unfortunate enough to try lubricating your rubber tread with petroleum based oil, then you have a big problem. Regular oil softens the rubber and makes it tacky.