1993 Dodge Dynasty not charging all the time

I just got this car and it was running fine for the last month. It sat prior to that for at least a year or so. I brought it home on a long trip (250 miles) and half way home the check engine & check gauges lites came on, I noticed that my volts were only showing about 10 or 11 on the dash meter, I assumed the alternator went and I was only running on the battery but I was close to home and took a chance. I got stuck in traffic and thought I was done, I put it in nuetral and revved the engine a little and the gauge went back up to 14volts. I made it home and shut it off in the drive and started it back up and everything was fine. I went to autozone and they checked it with their machine and said the battery is more than ok and its a bad alternator, not in stock. I headed to murrys and on the way it went back down to 10 or 11 v. They checked it and said bad battery - good alternator. I did nothing and headed home, all the way home it was ok. I called advanced auto parts for a third opion and they say it could be the ecm because it regulates the voltage, or the little black box mounted on the side of the alternator that is inline with the positive lead. I dont know who to believe, please help. I took it out again 2 times just around the block it happened once more and now its fine? Anybody heard of this. Its a 3.3V6 120amp
Thanks
Hank