Throttle body or idol valve?

I've been told it is either the throttle body or idol valve. When stationary, the car revs on its own up to about 15 then back down again, and it will continue to do this consistently whist idol, but it doesnt do it whilst moving. Further, the engine often cuts out easily when it gets down to a really low speed or stationary. When the car cuts out whilst moving this makes the steering very heavy and hard to turn.

In addition, the car often backfires.