rover 25 auto problem

[alc]

Hi I have a rover 25 automatic 2002 only done 21000 miles there is a problem with the auto gearbox. Everything is fine until the revs exceed 2000 at which point it leaps to 4000 revs and first gear is selected. I can only clear it by turning the ignition off for a moment and then restarting and the vehicle is back to normal. Does anyone have any ideas please?