High/Low idle

I call this surging and it can happen all the time, only when engine is cold, or when the engine is warm. Yours appears to be only when warm. Here is a list of things to look for:

vacuum leak - could be a hose cracked, broken, or loose
bad or plugged IAC valve - the Idle Air Control valve can be taken off and cleaned
air in the cooling system - with the engien warm crack open a bleeder on the cooling system (antifreeze not freon) Remove the air from the system. A possible leak in the system could be allowing air in.
Check the EGR valve
bad O2 sensor
bad MAF sensor

Start at the top and work your way down.

I am not sure why it fails to do it when in neutral, but generally it does not do it when the vehicle detects a load. What I know is that you have a sensor that is reporting bad information. The ECU tries to compensate for the information it is getting and it is wrong. So, that said, the sensor at fault is either failing to pass on good information or is gathering bad information. For instance, the temp sensor located by the thermostat could have a pocket of air sitting on it and thus the sensor is not able to read correctly the temp of the engine. It can just be a failing sensor as well.