Excessive power draw

I have a grand am with a new die hard battery cant remember exact cca's and a new 160high output alternator. The reason I got the alternator was sometimes when I had everything running(headlights,heater,system,1600watts 2 12s etc) the voltage would get really low and sometimes the car stalls out. I went to a shop and they said I need a new alternator so I got it but I still have the problem to some degree and the shop has no answer. When Im on the gas theres no problem but sometimes at red lights even with the system disconnected I still feel bogged down.  This problem has only been going on recently and Im not sure what to do, I was thinking a 2nd battery may help but Im not sure. Any advice would be great

When you say the problem has only been going on recently, what does that mean?  Did it work fine and then suddenly stop?  What changed?  You probably could use a smaller pulley on the alternator so it will spin faster.  Also have your voltage regulator checked out, as it may be failing.  But if things were fine for a while and then not, something broke, wore out, or was changed.  Figure that out first.Support the12volt.com