Voltage Issue!!!!! Battery or Alt?

So I put a new HO alternator on. I have the two batts in my signature. Also, Big 3 done. It was doing fine about 14.1 at idle. Then All of a sudden when at idle my voltage drops to 11-12 volts. Which is obviously bad. But then when I go faster the voltage increases to about 13.7ish. Then when I put it in Park (in idle) the voltage goes up to about 13 and stays steady. When in drive idling it drops again. Until, I accellerate. It is really wierd.

Thanks in advance,

Joe