Peugeot Auto over-revving in neutral

[nonsense]

I've got a 1997 peugeot 406 2.0 Auto (Just to brag obviously), and when i put it into neutral or park it idles at about 2000rpm, sometimes slightly roughly. When i put it back into D it's fine again. This only began this weekend after a 900 mile jaunt to cornwall. It may or may not have something to do with the fact this is my first auto, and on the huge long downhill sections i knocked the box into neutral and coasted, which according to the internet this morning;
a. might not actually save fuel
b. may have a detrimental effect on the car.

Whoops.
Could anyone help please? Have i broken it, and if so how? Is it likely to get worse or could i just happily ignore? or How do i fix it?

Cheers guys

[nonsense]

bumping up, anyone?

[TheEnd]

Sounds like you have an air leak or a torn intake hose somewhere.

[littleredrooster]

Possibilities include: Idle control valve, dirty air filter, air leak, exhaust leak, faulty air flow meter.

I'd start with idle control valve; it should work so that the idle speed remains the same when it is put in 'D' - i.e. it should overcome the gearbox drag. Seems that it is open or active all the time meaning a fast idle when there is no drag from the 'box.