My best guess is that you have the gears incorrectly shimmed, reopen the gear box and fiddle around with them. Every time that you try something new, put the top of the gear box on with out the spring or the screws and put you fingers on the bushings and gear axels, there should be a little play between the gears. Additionally, if you look in between the gears while the mechbox is still open, there should be a small space in between each.