The alternator was built to run the vehicle and have a small amount of reserve capacity. If you are drawing a crapload more current over the rated amount, it is going to start to rob current from elsewhere in the system, most noticeably the lights as it is what you can see. While a cap may be beneficial here to smooth out the peaks in the draw, it does not solve the problem. The problem is that you are drawing more than you car can produce. Bigger alternator time. The more electrical crap in the vehicle, the more it is going to draw.