http://www.precisionpower.com/amps/dcx1500.asp

A 1 farad cap is huge. I doubt that anyone said to use a 40 farad cap. a 1 farad cap for the car runs around $150 for a good one.

The purpose of the cap is to give you power faster than a battery can. Remember, batteries are meant to HOLD current, not to let it go. I don't know what your problems is but it might be a weak or too small battery.

I'd recommend talking to the manufacturer of the amp.