I had a problem like that a while back. The sub would just give out a random thump every now and again. As I remember the problem was noise coupling onto the coax cable. I don't remember if the solution was a higher quality coax cable, rerouting the cable or both.

Try rerouting the cable first making sure it's not next to any power cords or speaker wires.

If that doesn't do it then it might be some weird ground loop. I've had ground loop problems with other subs but not with my Outlaw.