Excuse me Gentlemen, but I found the info below on Range Rovers.net, and thought it might be useful for your idling problem. If not then I will get back in my box.
Idle Problems
Throttle Position Sensor: Problems such as fast or erratic idle or stumbling can be caused by a bad
Throttle Position Sensor (TPS), causing the wrong signal to go the ECM which in turn will tell the injectors to send the wrong amount of fuel. For example, the ECM uses the lowest position ever seen as the idle setting, and if there is any drifting occuring in the sensor it can throw the idle setting off.
Idle Air Control Valve (Stepper Motor): This does not cause as much trouble on the 4.0/4.6 engine as it does on the Classics, but can still play up and cause surging or other idle problems when it gets gunked up with sludge oil from the crank case vent. Malfunction symptoms that have been reported include stalling and near stalling especially when coming to a stop. Also hesitation before accelerating when operating the kickdown.
David Gomes reports that it is also called the "EL stepper motor". He points out that on the 4.0/4.6 it is in a different location -- and is held in place differently. It sticks out of the front side of the throttle body, and looks like a little can about the size of a 35mm film can. To remove it, unplug the harness and then remove 2 torx screws to remove the unit. Reverse to refit.
Cleaning the unit can often restore proper operation. David's method is to hose the pintle and seat down with some throttle body cleaner (aerosol) and wipe off the coking.
Dirt Buildup in Air Intake Plenum and MAF Sensor: This is another cause of syymptoms like erratic idle, stalling, cutting out at idle and when coasting up to a stoplight and stumbling when accelerating off idle. such. SOmetimes the "check engine light will come on and sometimes not.
Barry Higginbottom had this problem with his '95 Range Rover. All it needed was the air intake around the plenum chamber inlet cleaning. The build up of oily gunge around the throttle valve was sufficient to upset everything at idle speeds. A quick cleanout cured the problem. The Mass Airflow Sensor (MAF) has a passage the air takes separate from the main air flow. You can squirt WD/40, water, alcohol, and/or air through it to clean it out.
Alternator: Tony Sawyer reports that if the alternator is getting tired, the voltage/current to the coil can drop enough at idle so that the fuel injection system is getting a weak signal. This will cause misfiring. If you have several accessories running (computer, stereo, Engel fridge, GPS, ham radios, CB, etc.) it can reduce the voltage to the point where the engine will die. Once you speed up, the alternator kicks in enough juice to cover all your other goodies.