Have you looked in the manual? It gives instructions on how to clean and lubricate the rails that the mechanism rides on. They are part of the playfield, not the motor mechanism.
Secondly, if your belt is cut - a long length, not a loop - then you have to watch how you attach the two ends on the belt holder. Look at how it lines up to the pully wheel .. it should be roughly in line. I made the mistake of putting the two ends in incorrectly and it would fall off when it got close to the pulley.
Lastly, are the belts tight and "grippy"? New belts have a little tack to them so there's no doubt they will grip well. My older belts felt OK but kind of hard in some spots, had a couple flat spots, and were generally nowhere near as grippy or taught as the new ones.
I doubt your motor is the problem. can you share a video showing how much tension you have on the belt and how it aligns at the top and/or bottom of it's travel?