why do you doubt it is your cooling system? i didnt mean a/c or power steering. i dont have neither.
while your driving and while your rpm's are high, your engine gets very hot, but your radiator is getting sufficient airflow, allowing the cooling system to keep the engine at a safe temperature. but when you stop, that engine heat is still there, and the thermostat will switch the fan on, but there is still excess heat. the excess heat gets trapped in the engine bay, and the only air source for the idling engine is extremely hot air, causing your idle to drop a little. then when you start driving again and as your engine requires more air, the only air source available is still hot air, until your at a sufficient speed where the engine bay has proper ventilation and the radiator as sufficient airflow.
if your cooling system is not up to par in the summer symptoms like you explained in your OP could occur. to be honest with you the same exact s**t happens to my car on a very hot day.