Engine revving?

I have a citroen Xsara picasso 1.6sx. It has been fully serviced all its life. But over the last few weeks the engine revs up when the car is stopped. You can hear the engine drop revs then back up and few times then it revs very loudly until the engine is turned off or it is driven again. Had the sensors all cleaned but still it does it. It only does it when warmed up though, Also it stalls first thing in the morning.