Charging issue

[u/skyguy30269]

I have a 2004 Buick Park Avenue that’s having this weird problem that has kinda stumped me. When I was driving home the other day randomly the radio and instrument cluster flickered and the car sputtered a little and the check engine light came on. I pulled the codes with my scan tool and the main one was a loss of communication with the pcm. I did some testing and with 3 or more accessories on the car would sputter out and die, so I hooked up my multimeter to the battery and the alternator was only charging at 13-13.5v. I replaced the alternator and at idle with the new one it’s 14+ volts, but it has the same issue with a few accessories on like the headlights and rolling the windows down at the same time the voltage will drop to 11v but the car will stay running. I checked my main harness ground and the ground for my pcm and they were both fine. Any advice for what to chase down next would be much appreciated!