It sounds to me like the motor may be OK but you're having problems with something causing resistance.
Check the cables to see if there is any fraying of the wires causing them to catch.
Check if the window glass is connected properly to the frame -and where the four set screws are - if one of them is loose the glass could be moving about.
Check for any obstructions or broken parts.
If all that seems OK I'd spray all the moving parts with silicone lubricant and keep working it up and down to see if it improves.
Last edited by jonb.gt; 01-09-14 at 18:13.