if tune up, firing order, governor, carb, and other peripheral repairs dont help. could be a flat or worn cam lobe. pull the valve covers and watch the travel of each rocker arm as you crank the engine. you can do it running but its kind of messy. i have only seen this happen twice on the toyo product but it used to be very common on the 4.3 gm in other applications. could also be exhaust valve issues "recessed or worn seat" but thats a little harder to check. pease note that my suggestions only apply when you have done or checked all of the usual suspects.