alright, so im assumung that when you listening to music, the sub just randomly starts thumping? few things you need to check.
1 make sure that your rcas arent shorting out anywhere.
2. make sure that they are plugged in securely to the headunit, and to the amp
3. make sure that the amp is grounded properly, and is receiving sufficient power
4. ensure that all your speaker cables are secure, and arent shorting out.
if this doesnt fix the problem, then take the amp out, and try it in another car. if it still does it, the amp is most likely fuxxord