Car losing power when A/C running

[Gs400_843]

First off i have a 1998 Gs400 Every time i turn my a/c on somehow my car starts losing power, first the RPM gauge goes up&down for a little while then after a while radio starts fading in/out, then battery light comes on when in park. I done had my alternator replaced twice within the past 3 4 months. When i have everything off car runs fine but when i start turning things on it die down especially when a/c running.

PS:Just checked my alternator again the other day it was reading 13.9v but when i turn the a/c on it dropped to12v-12.7v...

I NEED SOME HELP PLEASE THIS PROBLEM REALLY URKING ME AND ITS TOO HOT OUTSIDE FOR THIS ESPECIALLY TO NOT DRIVE WITH A/C

[TrevC]

I would check the battery condition as it appears that any discharge is drawing down the battery supply. Check this with a heavy duty discharge test. Check also the earth leads along with the battery terminals for corrosion and security