Intermittent Charging Problem

I just picked up a '98 Dakota with the 2.5L 4 cylinder motor. When I first checked it out, the battery was completely dead, so charged it up with a battery charger with no problem. While driving across Arizona, all of a sudden I got a "Check Gauge" warning, and noticed the the gauge for the charging system was all the way down - no charge. As I continued to drive, various things started to shutdown...the radio, the other gauges on the dash, and finally the whole vehicle. I figured it was related to the battery, but shouldn't have the vehicle continued to run if the alternator was still good?
I put another battery in it, and the electrical gauge jumped right back up to the middle, near "14", but would occasionally drop back down and go lower towards the "8", but then jump back up to "14" while I was driving. I'm assuming alternator, but just want to make sure before shelling out the bucks for one.
Any suggestions?

sounds like alt failure voltage regulator is in the pcm
i would run engine with a volt meter on the battery and move or add side to side pressure the field connector at the alt while monitoring the voltage to see if you can duplicate the no charge condition this way