Engine misfire

Hello, I have a question for you all. The problem I am having is, at idle the engine misses ever so often. The engine will continually misfire. I have pulled the spark plugs and spark plug wires and could not find anything wrong. The spark plugs are clean and the spark plug wires are within range (checked with OHM meter). The van runs great going down the highway (no miss at all), but once you stop at a stop sign or stop light, the engine with start missing again until you take off. Once you take off, the van runs great. One thing that I have noticed is that if I have the A/C running and I stop at a stop sign or stop light, the van does not miss because the engine is idling faster because of the A/C being on. Please help and thank you.