Weird problem

   My brother in-law has a weird problem going on on his 97 Saturn SW2. Every time the bass hits the lights and the rpms go down. And this is only with the volume about a 1/4 of the way up any higher and the amp starts to loose power and the meter on the cap goes down to single digits.

   Now he only has one amp a Kicker zr360 and it's only pushing a single IDQ12. Now the car got a 105 amps alternator with a Yellow Top Optima and a 4 gauge Tsunami wire straight to a DEI 1.5 farad cap and to the amp. And the ground is also 4 gauge.

   Now when installed the system I didn't cut the power wire  to connect the cap I just removed the insulation on a  2 inch section of the wire and twisted the wire into a 2 gauge ring without cutting it to minimize the resistance on the wire in between the amp and the fuse on the front. The ground  wire is connected the same way to the cap and is about 15 inches long and it's bolted to the floor with a 1/4 inch bolt and a lock nut then undercoating spray on both sides.

   We tried the amp on another car and it works fine so it has to be the car. But it doesn't make sense because even with the engine off the battery should provide enough power to run the amp but the amp does the same thing. I'm thinking that it could be the ground on the car  and we have been contemplating the idea of running a ground wire from the battery to the grounding point where the amp is grounded instead of connecting it straight to the amp.

Now has anyone run into a problem like this before?

Or does anyone have any suggestion on how to fix it?

      I've tried everything you suggested and them some. I think is the ground on the car, because there are two ground wires on the engine compartment, one looks like a 6 gauge that goes from the engine to the battery and  another one that looks like a 8 gauge  from the battery to the frame. I haven't  checked the rest of the engine compartment for extra wires.

  All factory wires are connected to the side posts of the battery and the amp wire to the top post. And this is a Yellow Top which should provide more than enough power to run the amp with the car off.

  And no is not the radio I also tried an mp3 player straight to the amp and still the same.

   Maybe I didn't explained this right I would keep the factory ground but run a wire from the battery to the point on the car floor where I grounded the amp(or a point near it).  Not run a ground straight to the amp. It's got to be the ground because the power wire is new and so are all the connectors the only thing that is used is the car.