DrumrReagh13

ok so my cars voltage is dropping way lower than it should when im stopped. Ive got an amp and a sub, but that i feel is not the problem, when i stop at a light my voltage drops to almost 8 volts and when i go it shoots back to 14. some days it will do this and somedays it wont. could this be my voltage regulator acting up? and yes I have a capacitor, which is pointless anyways when u have "the big 3". and it was installed correctly by me, its my job everyday so i know that end is alright.any help would be greatly appreciated.