If you are driving around, and are below 13 volts, something is wrong. Try to hook up a voltmeter to your cigarette lighter socket. If the voltage hovers around 12 volts but then suddenly jumps up around 16 when you rev it, something is loose.
Ususally alternators die altogether and don't become intermittent. Also, if the small stator/regulator wire is loose, it would cause these problems.
HTH,
BP
