I just purchased a 2001 GMC 2500 hd with only 20,000 miles on it. Looks and drives just like a new truck. At idle it seems to run smooth and then stumble abit, Then it returns moments later??? How it corrects itself is a mystery. Also after the truck shifts out and low rpm only slight throttle it seems to have an engine miss more like a lean stumbling feeling. All other positions runs great! I have replaced the gas filter, air filter, spark plugs, wires and cleaned the mass airflow sensor. No difference???? Also no codes?? Any ideas would be very much appreciated. Thanks!!