Our picasso is exactly the same. Its an 2002 2.0D and happens occasionaly at start up and feels ok once driven and pedal begins to depress. My wife drives this car and I am worried this will happen during a journey!
My guess on this is that a relay to start the passage of servo assisted vacuum is working intermitently.
If you find what it is please let me know. I will do the same also if I find the issue.
Rob
December 2008