Unforntnately, this common problem as nothing to do 'dirt' or comnamination of the wheel hub trigger wheels, BMW's unlike MBZ were never susceptible to this. The most common failure though they will be quite intermittant at first is the ASC Control Unit which screws to the Hydraulic Unit mounted on the R/F wheel house and is easily replaced. Fault Codes may include wheel sensor failures but actual testing [which is part of true diagnosis, not dart throwing} will show perfect output patterns] and when they test good [which they always have in my experience] replacement or 'cleaning' is a waste of time.
Hope this helps...........................jeff b.