Amp and Cap Drop Below 12 Volts

First when i would turn my system up to a certain level the voltage would drop to like 9 and my 4ch and mono would cut out and when i turned it back down everything worked fine. My friend told me it was my battery so i went out and bought a optima yellow top. Now  at low volumes its like 13.5 but when i turn it up it still drops to 9 but it doesn't cut out. What did i do wrong that it's not a steady 12.Headunit : Alpine CDA-9855

Speakers : Polk momo 6.5 Components

                     Polk DB 6x9

Amps: Infinity 7541a 4ch

      Alpine MRD-M301 Mono

Subs: 2x 10" mtx rt1004

Start by checking your grounds.Top Secret, I can tell you but then my wife will kill me.

it's a 97 accord ex. mono is 350rms at 2 ohms and the infinty is 111 x 4 at 4 ohms (which i highly doubt but thats what it says) thanks in advanceHeadunit : Alpine CDA-9855

Speakers : Polk momo 6.5 Components

                     Polk DB 6x9

Amps: Infinity 7541a 4ch

      Alpine MRD-M301 Mono

Subs: 2x 10" mtx rt1004

What is the voltage reading at the battery compared to the amps (across the amp power/ground terminals) under these conditions? My ex once told me I have a perfect face for radio.