You are correct most likely the motor is over heating. The things that will cause the motor to over heat are:
1. Low voltage from the house wiring. Are you using an extension cord? Try moving the treadmill and plugging it into another outlet without an extension cord.
2. Does the cord get warm at the plug? You might have a worn outlet in the wall.
3. Dry or worn walking belt or board. Lift the belt and feel underneath it. If the belt is dry you can lubricate it. The belt wax part number is 134591 and it comes with instructions. If the walking board has rough places on it you would have to replace it..
4. If the walking belt is too tight, the motor will over heat.
5. If the user is over the weight limit of 250 pounds.
If the motor is not overheating there may be a problem in the motor, motor controller, console, or wiring. These things require test instruments and you should schedule service and let a technician diagnose and repair the problem.
The belt lube can be ordered from this link.
If you do not feel confident repairing this problem yourself, then you can have it repaired at your home by a Sears technician. Here is a link for the website:
Sears Home Services