hi i have the same problem
proform 590hr treadmill
turn on machine all lights come on the belt trys to turn but will not run
looked inside no loose wires ,poss motor or lcb broke .
what do you mean when you say set the i/r comp to 85%duty cycle
not sure what you mean ?

p williams, February 2008

on lower some lower boards there are three dial pots.
IR, Low speed, High Speed.
IR is for body weight compensation
low and high speeds are for what they say.

But I don't think thats your problem.
Stand to the side of your treadmill and push your belt with your foot. Then start the treadmill. If you have worn motor brushes this can kinda jump start it and it may move the belt in a jerky manner. You would need to replace motor brushes and clean the commutator.
If on the other hand your treadmill doesn't start and stops when you quite pushing. Then your motor brushes are totally burnt or you lower control board may be out.
. For further help You can contact me here,
www.mindbites.com/person/214-Mark
or markepeterson@austin.rr.com

Mark Peterson, February 2008