theslepper

I had recently change the stock alternator that went bad to 160amp alt, today im driving to work and i noticed that the gauge from the battery was dropping, within 10 min the car turn off completely dead. it took it to the mechanic he check the alternator and was producing 12.6 at idle. I have a yellow top opt. HID and a stereo with 2 12`s.
the alternator is less than 8 months old.
Could there be something that is messing up with the alternator? should i upgrade to 200amp?

thanks