So on my mums MK4 Fiesta the throttle has all of a sudden just got really funny and thought i would ask you lot at PF your opinions!

When you put your foot on the throttle and apply a bit of pressure it starts to build up the revs as normal but if you put your foot down straight away, it doesn't rev up as it should it bogs out to below 1000 RPM and then starts to build up! Also if you put your foot down all the way before it gets to 3000RPM it will stay at 3.5-4000RPM and just rev at that max but if you do it slowly it will go all the way to redline!? Any help would be appreciated