Try removing the throttle cable and see if it still occurs.
Check the spring return action of the throttle body is free to close under its own effort.
Had this on my BMW and it was simply I'd routed the cable wrongly round the engine after a bit of work.
Used to fix quite a few vw transporters with a sticky throttle cable simply cos it had come off a few clips round the bay etc.
Caused all sorts of issues!!!