[diverrick]

If you'll get a volt/ohm meter, try looking at the field wire at the alternator, and seeing if it is following the same as the gauge. If it is, you have a regulator issue, if it is not doing the same thing, then it is either the gauge or the alternator, Next check the output of the alternator.
Alternators don't usually work one moment and then not the next. The either stop working altogether or get low voltage due to a bad Diode.
You can also use a jumper across from the battery to the field, which should give you full unregulated power. That'll show if the alternator is working properly.