Might be running your LSN too lean. Your wheels shouldn't be spinning when they are lifted off the ground. You want the engine to idle fast enough that it doesn't cut out, but not much higher than that. Read up on the engine break-in in the first post of this thread
http://www.rctech.net/forum/offroad-...ng-engine.html. You are actually currently breaking in your engine with too narrow of an idle gap and running your LSN too lean. As for your spur gear stripping, you likely had the mesh set too loosely. This allows the metal pinion gear on your clutch bell to spin freely enough to chew up your plastic spur gear.
Another piece of advice: If ever your engine revs up out of control for some reason and you need to stop it (seen this happen when someone's receiver battery got disconnected mid-race), use either a thick rag, handle of a screwdriver, or an exhaust occluder (if it came with your kit) to plug the exhaust tip of your pipe. This will make the engine safely stall rather than it over-revving and toasting your engine.