While driving my '78 Spider tonight, I noticed that my generator light starts to come as as I hit 4000 rpm, then glows more brightly as I exceed 4000 rpm. Once I drop below 4000, the light goes out. (I never noticed the light coming on during the day though it's possible it's coming on then as well and I just haven't noticed. Otherwise I'm guessing running the lights at night is the difference). Should I assume an alternator issue, or is there another possibility? If alternator, can I just service it (worn brushes) or is the only fix to replace it? Thanks.