yep! i can confirm the above! i have two power caps and they are totally useless, worst money i have spent, only a high currentflow battery and/or alternator upgrade will do me now!
but also as said above, assuming the equipment is in the car in your sig ( and possibly even if its not!), you shouldnt need any of the above. you are either... (A) in need of a new battery (not a particularly expensive high performance battery though, just one fitting to the engine size) or (B) running the amp into distortion causing excess current draw. or (C) need to upgrade the wiring or fuse, be it to the amp +ve or amp ground or the car earths (from the battery to chassis, alternator to engine block or engine block to chassis). what fuses are you using in what thickness wire?