Adding a second battery...maybe?

Hey all, recently ive been having a problem with my amp and sub in my truck. I installed them a few months ago and everything has been working fine, but lately its been acting weird. When i turn on my radio everything works and the amp is on and the sub is kicking, but at random times like in the middle of songs the sub will just stop, and then maybe start back up again...am i missing something really simple? the only thing i can think of is maybe the amp isnt getting enough air flow or something because its behind my back bench seat. Could it have anything to do with a battery? ive been thinking about adding a second one but i dont know how to do that, any help is appreciated thanks!