I haven't looked into the efi system on the Xsara but your problem sounds like the idle control actuator as mentioned.
If the car is still in warranty jsut take it back and get it replaced.
This was a common problem with Bosch LH2.2(and similar systems) jetronic and is a symptom of a failing idle control motor. If you can remove the control motor and clean it with carb/throttle body cleaner it may work better for a while.