Picasso revs dipping

Hi i have a xsara picasso1.6i 8v on a 2001. When cold its fine but after 5 or 10 mins when pulling away in 1st the revs dip and u have 2 rev it hard 2 get away. If you turn the ignition off and back on it clears the problem for 5 mins. Any help would be gratefully received thanks.