Just worked out the problem with mine, cracked idle bypass hoses - some of them are a bit fiddly to get to, but for the moment i've just covered them in duct tape until BMW open here in oz again on monday morning.
Its fairly easy to miss it and i was just about to start ripping the throttle body apart and then i saw the cracks, taped them up and seemingly its running fine again... dont know how i missed them the first time!
Hope it helps!