Renault Clio wont rev

Hi,
My daughters 02 clio 1.2 has the following problem,

It starts and drives fine this can be 5 miles 100 miles or 50 yards.

Suddenly it loses power and although it does not cut out completely it will not rev up.

If you can remember the old carbon ignition leads used to cause a similar situation when the became old!!!

If you dip the clutch and frantically operate the throttle eventually it will pick up and rev and go back to normal until the next time

Any quidence please this is driving us both mad!!