Your remark about the alternator warranty suggests you are running a "remanufactured" alternator from a parts store. They're usually junk that works poorly no matter how many times they get replaced free of charge (as it were). You can get a
new (not "remanufactured") one from
Old Car Parts Northwest.
Make sure the alternator, regulator, and battery are all bonded on the same ground plane. Run a new 14ga wire from the "GRD" hole on the back of the alternator housing to the voltage regulator base, and from the voltage regulator base to the battery negative terminal. See
here and
here.