Mad Rat Won't Run

[Amazing]

Hi,

I per my previous my Mad Rat (son's Christmas present) won't run. I ran it for ten minutes and it stopped. The motor won't turn and makes a noise as if it is strained. I took off the gear box and the motor runs fine on its own. i stripped the gear box and that appears fine, no stripped gears etc. It worked off the car intermittently but not when i re assembled it.

any ideas on the what the problem could be ? Could it be the speed controller ?

thanks