I have a 2 year old 32" Exmark Viking with a 15 hp Kohler engine. The engine seems like it wants to die (RPM's go down, backfires), and sometimes actually does die. This condition usually happens when the engine is warm and either the PTO is engaged or it is in gear (moving). The machine is greased properly, the engine oil is fresh. I have replaced the spark plug, the fuel filter and the air filter (and I keep the pre-filter clean). Any suggestions before I take it to the dealer to have them look at it ?