First of all, make sure that none of the driving rods are bent, and none of the hex bolts, eccentric screws, etc. are loose. Can you turn the wheels smoothly by hand for one complete revolution? Look for chipped gear teeth, or something binding in the gears.
Next, how much slop is in the gears and axles? If you turn the center wheel back and forth a few degrees by hand, do the intermediate gears rock noticeably on their studs? Are any of the driving wheels loose on their axles? These locos from the peak years of toy trains often saw a LOT of use in their first few years of life. It might be time for heavy repairs. Find someone with a puller and a press. And most of all, with experience doing this kind of work!
