1999 STS charging problems

I got a 99 cadillac sts with 160,000 miles and the 4.6 northstar motor. The problem I've been haveing is with the charging system of the car. After purchaseing the car I found out the alternator was bad and recently replaced that but when I replaced it still did not charge the cars battery but did provide the power after start up to run the car. Monitoring the voltage I notice it would operate at between 12-16 volts depending on the RPM of the engine whiched seemed to be unusually low/high. Usually this car would run about 13.5 volts with minor fluctuation. Also because of the high voltage it would trip warning lights including the abs, traction control, service engine soon, security, and also cut out the rear audio amplifier. This problem also caused the alternator to burn itself out in a matter of a few days probably due to the high voltage output. This would be the third alternator within one year. I believe the warning lights have only appeared because of the voltage/charging problem. I believe this because I disconnected the alternator from the car itself under the hoods fuseblock and reset the cars computers. I ran the car all day only running off the cars battery (with alternator disconnected) and not one warning light or problem besides the battery indicator. I also replaced the PCM and DIM computers thinking this may have been the problem but did not solve it. Any help would be much appreciated.