I second what jbaj007 said.
Voltage regulators can be had for as little as $25 in the US. If you haven't replaced it or the alternator in a long time, it's probably worth the gamble. At least carry a spare with you until you sort out the problem. Very easy to replace with a screwdriver and access to the alternator from below.
Also a good idea to replace all the fuses if you haven't done so in the last 10 years.