I have experienced this problem on a friends X5. The same stories as above no one was able to fix it or have an answer that actually made sense.

In the end it turned out to be this..The Automatic control ECU sits at the back of the black in the engine compartment on the right hand side under the scuttle pannel(Black Plastic thing) It is right under the open bonnet vent that allows all the rain and jet wash water to run away through. his box was not completely sealed and we found condensation on the blue ECU plugs that wre causing electrical tracking accross the terminals which were shorting out or frying as the dealer said. i quick squirt of WD40 and a dry out and then a water sealant on the actual black box and it has been 100% now for nearly 10 months.

I hope this might help you !!