Alternator quality is a crap shoot. Any parts store like Napa etc. is going to be dicey. Rebuilt Bosch 7 or 8 years ago was good, still reman'ed in Germany and was more expensive than the new Bosch which was made in China at that time. The best thing to do is go to a local rebuild shop like Effduration says, if you have one near-by, or try to get one that you can get some confirmation on the quality/reputation. It's not uncommon to have multiple bad crappy alternators.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)