My last sub did the exact same thing so I can pretty much rule that out as the problem.
All wires are secured tight and have a solid connection, though most of them are crossed and in a pile in the trunk. My 6x9 amp and sub amp are both flopping around loose on the trunk floor, but no wires can really touch. I tried smacking my amp around today while it was playing to simulate me slamming on my brakes and I couldnt get it to do what happens when I hit the brakes. Anyone have any ideas? lol.. I took my amp apart yesterday and tried to feel for a loose component and didn't see/feel any. I'm stumped...ugh..