Apparently me and Matt agree
I honestly can't fathom your hardware utilizing that much power from the alt. I wonder if it's on it's way out the door like Matt suggested earlier?
Also, miscalculation earlier. I had thought that perhaps a cap wired in with the amp would help with it's possible inept power platform. However, caps will only help if they're part of the actual power supply design. Once you move caps outside of that system and wire them as part of the electrical system they do practically nothing to help your system. If you have power problems, new alternators are the only way to go. But you must upgrade the necessary wires to utilize the resulting amperage.
So if you've done everything correctly in terms of solid connections and using appropriate wire sizes & lengths your only other option is to put in a new OEM alt. or an H.O alt.
