sounds like a stripped bevel gear in the gear box. take the gear box tops off (probably the rear one, and check the gears. the smaller gear will probably have very rounded teeth and be slipping against the diff gear.
common problem with lazers. make sure you shim the diff gears a bit tighter than the manual says. move one of the shims from the inside to the outside to tighten the mesh, shouldn;t have a problem after that
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