1995 buick park avenue HELP

i just replaced the alternator and the powertrain control module “PCM” and the voltage still jumps. from 14.1 normal

to 19volts and then when i stop or idle it goes all the way down to 10 to 11 volts and the rpm drops below a 1000 if it not the pcm or the alternator i dont know and the code reader just reads low voltage