Sorry, don't know the truck. But there are a few things you can check. Engine bolts, are they all extremely tight? Gear mesh between clutchbell and spur, is it set right. should be minimal slop. Do you have a cracked brace somewhere allowing chassis flex? Do you have a spur slipper? If so, is it set to tight and not allowing slip?
I have the losi xxx-nt and have had issues with the spur periodically and each time it was one of these things above.