Sounds like a voltage drop or your clipping the amp . Have somebody that knows what their doing check it out before you get a new battery or alternatormoney isnt an issue for me, would putting a new battery/ alternator in there help?
Alternator isn't going to be necessary at this power level, and you might as well keep the cap, it can do some good, but not for fixing your current problemhow bout a new batter/ bigger alt and i take away the cap/ power cell