I have had this problem for over a year its not the disk BMW told the previous owner it was the weather, a climate thing, what utter CR*P. The system drops off the IBUS at some predetermined point of its own making. I am determined to get to the bottom of it and have had several discussions on the Bimmerboard. The temperature may be a factor as when the air con gets the cab cool enough it works, but that suggests its not the MK4 in the boot. Good luck I am still hoping!!