charging problem 97 cherokee sport

I HAVE A 1997 JEEP CHEROKEE SPORT, 4.0 ENGINE, 152,OOO MILES. MY BATTERY IS NOT CHARGING. I HAVE TAKEN OUT THE BATTERY AND THE ALTERNATOR AND HAD THEM TESTED. THEY ARE BOTH FINE. I REPLACED THE BATTERY CABLE ENDS, NO HELP THERE. ONE AUTOPARTS STORE TOLD ME THAT THE VOLTAGE REGULATOR IS IN THE ALTERNATOR AND CAN'T BE CHECKED. ANOTHER PARTS STORE TOLD ME THE VOLTAGE IS REGULATED BY THE COMPUTER. THE JEEP STARTS AND RUNS GREAT WHEN I USE MY CHARGER TO CHARGE THE BATTERY. THIS WILL ONLY GET ME ABOUT 1O MILES. MY CHARGE GUAGE GOES UP TO 12 VOLTS WHEN I START THE JEEP AND GOES DOWN TO BELOW 9 VOLTS IN A MATTER OF MINUTES. SOMETIMES THE CHARGE GUAGE WILL GO UP TO 14 VOLTS AND WILL THEN CHARGE THE BATTERY. SO WHAT DO YOU THINK COULD BE CAUSING THE PROBLEM.