NO, its not a good idea to get a capacitor when u really need is a Alternator Upgrade. a capacitor will put more weight on ur stock alternator's shoulders.
A cap is just like a battery. So instead of charging one battery( the battery under the hood)now the alternator will have to charge TWO batteries (Cap and ur battery under the hood)
So to solve the problem Get a High Output Alternator. And do the BIG 3 up grade.