You need to check the system voltage with a load on the alternator. Turn on the AC, headlights, check the voltage at idle. Should be around 12.5-13. If below 12 then beware. Run it up to about 2000 RPM. If the voltage does not rise to over 12.5, then your alternator is dying. The slip rings on the rotor do wear out. New brushes can sometimes take a long time to seat on the slip rings. It's 17 years old. Time for retirement. BTW, the stator windings do fail. They can develop internal shorts and lose some of their capacity.
1987 BMW 325