Thought I share my fix for dreaded surging Idle on my 2003 R53 with 155K. Was getting the following codes P0507 and P1109 with idle jumping from 600 to 2000 RPM. After much troubleshooting found the issue to be cracked charge pipe causing the vacuum leak and idle issues.
Considering it's location and age of the car, can't see mine being the only one to have failed. Was not too hard to change, put car into service mode and curse and swear for 30 minutes getting the pipe back into place. Took 2 hours total and car is running normal once again, hope this helps someone with a similar issue.