Its a Renault Megane (2000 so old style), and car is abroad at the moment so I'm trying to help over the phone!

basically, quite frequently the revs will raise at idle to about 1100rpm (from 700ish) and run very lumpy.
Also when driving, often the car loses a lot of power, again feels lumpy and doesnt really accelerate at all, then all of a sudden it will cure and lurch forward.

happens when engine is hot or cold, but seems a bit better when warmer.

To me this sounds like its running on 3?

Are there any other quick things people can suggest to check? Im getting more details later but for now thats the obvious cause?
No engine management light on either if that helps
Cheers all