Your options are:

1. try swapping in a new voltage regulator from Valeo or Bosch..
2. Get your original or Genuine alternator rebuilt by a reputable shop
3. Buy a new or rebuilt alternator.

Be careful on buying new or rebuilt. There is a lot of junk out there. Don't go cheap. Bosch's rebuilt business was sold to an Asian manufacturer a few years ago.

I like option #2...and there appear to be a couple good rebuilders in Denver area.

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'99 E39 528i 5-speed 130k
'06 325Cic auto 115k
'05 X5 3.0 auto 173k-SOLD
'03 530i - manual swapped - 430k miles-SOLD