engine revving when coming to a halt or stationary vectra 1998 1799cc?

this is an ongoing problem, my car has been into the vauxhall main dealer and has had all the throttle ect cleaned out and new sensor and still the car is revving up to about 1500 revs, was worse before went up to 3000revs, I am at a loss, the main dealer has said they have exhausted all avenues but I think it could be the cranshaft sensor, could anyone enlghten me, it this a problem with the vectra of this year and what is causing it. The car itself is immaculate and has never missed a service. Please help!! Vectra LS 1799cc 1998