Strange cold idling behaviour

My Range Rover (3.9 V8), when started from cold, will rev itself to about 2000rpm and then drop back to about 800rpm in the space of a few seconds. This continues until it gets warm, whereupon it stops. It is most noticable in neutral but it also happens if running in gear on tickover which is annoying because it makes for a very jerky ride. Occasionally it will rev (by itself)to a steady 2250rpm when the clutch is depressed, hold there for about ten seconds and then drop back to its' normal idle speed.
I'm new to Range Rovers and am guessing that this is not an unheard of problem.

Does anybody know the cure for it? I'm reasonably handy and do the vast majority of work on my cars myself, so hopefully I'll have no trouble doing it.

Cheers,
JR

That could be a couple of things (as ever). I think I would start with taking the plenum off and giving it a good clean then re-set the base idle speed. Also make sure the stepper motor (idle air control valve) is working properly and that there are no air leaks in the induction system.

You might also want to make sure your air filter is clean. I also seem to think that you should check you have a good strong battery and that your alternator is in good shape.

Hope that goves somewhere to start - good luck.

Could be you have a vacuum pipe off or leak somewhere, that could cause weird idle problems like that. Open the bonnet with the engine running and listen for sucking/hissing from around the intake.

Thanks, fellas - will give it a checking out over the weekend.