2001 Chrysler Town and Country heat problem

While sitting still the temperature gauge get up to half way and the heater puts out good hot heat. While driving down the road the temperature gauge starts dropping and the heater starts putting out cooler air. I have replaced the raditor cap, thermostat, flushed the system, and the water pump look at. Also replaced the pcm because it was popping up a code saying that it needed replaced. What can I do next to try and fix this problem. Thank you