yes mate its more than likely causing the issue...might be best to invest in a voltmeter and putting it up front somewhere where you can keep an eye on things when your booming...

A cap will improve things but they are a temp fix and not the best way of going around the problem...the best fixes for you would be:

1) uprate your front battery with something from a van or a diesel (transit battery should fit if its a saph)
2) do the big 3...uprated earth points from engine to chassis, battery to chassis, and a minimum of 4g running from alt to battery
3) invest in a multimeter and run it off the bass amp...when/if it drops below about 12.4v, turn the volume down or you will run into trouble

failing all of that, if you are interested in a power cap, find the biggest one for the money you want to spend and use that....but caps dont last for very long before they need charging to...

if there is anything else i can help with just give me a shout as i supply/install audio equipment for a living

bassboy