My 1997 Z3 1.8l is sporadically running rough and 'conking out' (will restart by dropping into 2nd going downhill). Was doing this before its recent service at Quarry Motors Sheffield (~35k on the clock) but disappointingly continues to do it now. No clear pattern, it happens whether hot or cold, perhaps associated with coasting down hill (flooding?). It's clearly only firing on one or two cylinders and I can usually clear it by running the revs up. Any suggestions appreciated, thanks.