how can I fix my Ford Focus 1.8 zetec (2002) from cutting out ?

Hi, the problem occurs when revving the car hard. When the revs die down to idle speed, it doesn't idle and just cuts out, i.e, revs drop to zero. It doesn't seem to have any trouble when slowing down normally when driving and has not cut out on me. Only when reving. I have tried a new throttle position sensor - with no result. Any ideas as to what may be the problem?