Home theatre subwoofer (by 2 users)

Sounds like you have a loose signal lead. Check to see if this cab be tightened if not replace the cable as this could be the cause of the problem. Over time the leads/plugs do loosen especially on subs as they vibrate so much. If it's not this then it's probably something internal to which I am reluctant to advise you on looking at yourself unless you are pretty proficient with electronics :P