[nathan_hui]

I would definitely look at the gearbox. If you can, simply replace it. Otherwise, do what Eric says. If you can't figure out what the problem is, try replacing all of the grease. Make sure that there aren't any dings on the gears. Make sure the bearings are good. Since your problem was (according to your description) intermittent, I doubt it's the plates, but now that it apparently isn't intermittent anymore, Ankit's problem may have a lot more commonality to your problem than I initially thought.

As you are replacing the gearbox, run the CIM motor separate from the gearbox. It should spin up pretty nicely. If it doesn't, the CIM's shot. Doesn't mean the gearbox isn't shot too, so keep checking both.