206 Revving On Its Own

Don't know if anyone can shed any light on this one.
I've a 2000 206 (high mileage, recently bought) that's acting possessed! I've got a few problems with it, but the most prominent one is this
revving up on it's own! I can literally start the car, get her up into third and sometime i never have to go near the gas! If I leave it in neutral for a couple of minutes before i go off anywhere it's sounds like it's being revved up and sometimes very hard. It comes and goes and after maybe ten minutes the cars fine.
I've had it checked out several time and no one can work out what it is. It only seems to happen when it's started off after being left undriven for a few hours. I really have to get it sorted soon before I end up in the back of a micra some morning! If anyone could suggest anything to get checked out or has had a similar problem!!! Please!!!
Cheers!
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This could be an air leak on your inlet manifold arrangment. The problem goes away when it gets a little warmer and expands closing the gap.
Petrol or desiel?
G.

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