I recently bought a 2000 version of the ktm 50 pro jr....it ran ok for the first few minutes but as soon as it got hot it wouldnt stay running..the guy I bought it from said it fouled the plugs easily so I bought a new plug, cleaned the carb and bought a new air filter....after this it ran beautifuly for about 15 minutes (until it got nice and hot...then died again and wouldnt re-start....is this a top end issue? please help....