I experienced the same in my '83 320i (150K + miles) - beginning with the first (re-mfg.) replacement alternator I had put in and also occurring on another of the subsequent 7 (!!!) that it took to get it right. Each time it was the voltage regulation diodes inside the alternator that failed. I finally had a dealer go thru the car and verify that nothing else was wrong. The good news is that once I got a decent alternator from BMW, all problems ceased (>3000 miles and counting). Good Luck!