Belt slows and stops then starts back up at any speed

It probably has to do with the belt overheating the motor and irritating your controller. Try replacing the walking belt.

Have to check the speed sensor. Either the speed sensor is faulty or the can be dust or speed sensor alignment.
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Hello,
I would think you are on the right track, possibly a belt, I would also check all the rollers or idlers while you are replacing the belt.
Enjoy and best wishes

Have you lubricated the the running deck? The lack of lubrication can increase friction thus causing the current over load circuit to activate thus shutting down the unit to prevent further damage, Go to your local hard ware store and purchase silicone spray. Apply Lubricant by lift one side of the running belt spray contents length wise starting at motor shroud to the back end repeat this process on the other side. If this unit shut down again then you will have replace the running belt and possibly the deck

unplug unit remove motor shroud check your speed sensor by the front roller /pulley inspect and see if sensor is about 1/4 to 1/8 in away from pulley, most motor control board have L E Ds look for an amber light then manually move the belt and see if that light flickers if it does not replace it. also could be the the board but sensor is cheaper to try.

I believe under the motor shroud on the circuit board their should be 2 pots 1 adjusts torque 2. adjusts speed try adjusting them and see if that take care of the problem if not. worse case it might be the drive motor.

proform 625 belt slips cause

Check both belts to be sure they are adjusted and not slipping.Check the underside of the walk belt to see if it feels frayed in the center as compared to the edges, the center always wears more than the edges a really worn belt must be replaced. if it is nor worn there is a wax lubricant for proform treadmills that can be purchased at retail stores that sell treadmills.Sears, dicks sporting goods and also treadmilldoctor.com are good places to buy it.

Take off the plate around the shift lever between your legs and take a look at the belts. Most likely they will be in poor condition. You can even leave the trans in neutral, start the engine and move the speed selector through the gears and observe the belts in action. They do the same in neutral as in gear when you let out the clutch with the engine running. You should be able to see what the problem is, It will either be one or both of the belts or possibly the needle bearings in the vari-speed pulley itself,

There is a variable speed pulley that is a double pulley....one belt from engine to pulley and one from pulley to transaxle....remove both belts...pushing in the clutch will help with removal....remove o ring on top of pulley and pull up......the middle piece of metal is stuck....this piece is meant to slide up and down.....put the middle piece of metal on your pulley in your vice...wd40 the hell out of the shaft on pulley .....grasp the top with one hand and the bottom with your other hand...work the stuck pulley free.....put lithium grease on shaft...reinstall....done