according to your sig you are using x10 with your ac3, right? A quick way to test if this is your problem is to plug anything else into the same outlet, if it stops, then that is your problem. It is called Local Control in your x10 box, and you have to sever a wire to fix it. See neptunes site for help below.
http://www.neptunesys.com/hardwareFAQ.htm#LOCALCONTROL