most likely your slipper, it's the assembly that holds the spur gear, there is a nut on there, tighten it, also to test it hold the truck, push the spur gear either forward or back, it shouldnt slip, it should be very hard to turn it, if it slips, tighten the nut more, if it isnt the slipper it's the gear mesh, if your spur is ok then post it here and we'll go from there