[Warren Boudreau]

If the gearbox is assembled correctly, as was suggested in previous posts, then start of by removing the motors and turning the wheels by hand to see if there is any binding through the gearbox. Tweak the gearbox until all binding goes away. If those are 12 inch wheels, they ought to be spinning only 2 or 3 revs per second. 3 revs per second is about 9 feet per second, just to give you a ballpark estimate of speed. If those wheels are turning faster than that, your gear ratio is too high.

Good luck. We've all been there at one time or another. Just don't panic and start blaming anyone. Just concentrate on diagnosing the problem one piece at a time.