2000 Chevy Silverado Tries to run hot

Hi, we have a 2000 Chevy Silverado the engine is a 5.3. It seems to be trying to run hot but only sometimes. I replaced the thermostat, fan is working, hoses seem fine and both are hot after the engine runs a few minutes. If I crank it up and leave it in park it does not do it, but if I put it in gear it will get up to 3/4 hot then I turn on the heater and the temp will go down. No water is leaking from underneath and it does not appear that the water pump is leaking. Please help, what can it possibly be???