I'm somewhat of a nitro novice, but I think if its hard to start, or runs fine cold but stalls when it warms up you made need a rebuild. The engines are pretty simple to take apart and a piston and sleeve kit is only like 60 bucks. I'd rebuild it.
Don't rebuild it. Traxxas has a motor exchange program just giive them a call and they will explain. If it was me I would find a aftermarket motor like a picco.