1999 Volkswagen Golf Idle / throttle issues

The problem is that when releasing the throttle and depressing the clutch to stop or change gear for example, the engine no longer decreases the revs slowly in a controlled way, and instead the revs plumit very fast and before I've had a chance to change gear it's wobbling around 500-800 rpm. This has a knock on effect with the power steering which I presume is powered by engine revs. My understanding is that although the air intake is linked to the throttle pedal by cable, there should also be an electric control to help idle properly and to prevent the revs from slowing so quickly? Occasionally when releasing the throttle from approx 4k+ revs, the engine revs slow so quickly it actually misses the idle speed all together and cuts out, but most of the time, gets to 400-500 then wobbles back up to 800.