The part I've bold-typed is what troubles me about the original post.
If you know these two items are problematic wouldn't it make more sense to address those issues first (as they are real problems and need addressing) rather than attempting to put a band-aid on a broken leg with a capacitor?
If you've not upgraded your "Big 3" then I would suggest doing so. See the stickied tutorial about it if you're not certain of the process. Easy to follow step-by-step is there for your use. //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif
Following that a higher CCA-rating battery could certainly help and an HO alternator if your current unit isn't up to the task of supplying current to both your vehicle plus whatever you've added to it that requires energy.
If your charging system isn't up to snuff then adding a capacitor would most likely exacerbate the problem rather than help it.
