I would post this in the Stereo/Mulitmedia section but barring this post getting moved......
You have alot of load for your tiny little alternator. It might be max rated at 65~75 amps I believe, maybe bigger if you had A/C setup. (If anyone know the true specs please interject - I'm just going to use this for a comparative reference
) The 4g15 engine is meant to idle at very low speed and the alternator doesn't start giving you any real good charge - like stereo's NEED to have, until you start revving the motor up. The alternator has a winding/diode {tri-w or the other
) setup that is meant to give better output in the high RPM range not essentially down low, down low is another setup altogether for cars that can't rev like ours 
Getting a rewound, higher amperage alternator is probably your best route to start, but having said that, I always tell people with any real system that pounds, install a capacitor. Why a cap? stress relief for every thing in the electrical chain - it is like having an instant battery tap right at the amp and thus semi reducing the impact of their load (install positive within ~9 inches of the amp)
http://www.cardomain.com/memberpage/488838/2
This shows the setup I had just for ref... (4x10" subs in med. bandpass box-shook itself apart!)
When you install the capacitor you can actually see a difference in the way the alternator is hit every time the bass sucks power from the front- less needle bounce if you have a volt guage installed.
Berserko installed a rewound alternator on his ride a while back ....{hope he doesn't mind the reference
) just gonna search for a link to a site with alternator replacements/rewinds...http://www.4alterstart.com/pages/store/ ... lternators
Check locally for alternator/starter rebuild shops and get prices and warranties for their work.
This has all been IMHO
