I have a vintage Kyosho Outlaw Rampage with a ball differential that keeps slipping. I've dissembled, cleaned and greased it but it still slips. The differential consists of a center gear with 12 recessed ball bearings. This gear is sandwiched between two other gears which drive the rear wheels. the outer gears have a metal face that rub against the ball bearings. the face is smooth with no grooves. No matter how tight i tighten the differential, it still slips and by slips i mean it will barely turn the rear wheels and stop at the first sign of resistance... Any thoughts?