Ghost in the machine?

I need help. I have a nice home theatre set up. I have a Classe cav-500 5 channel amp running two b and w nautilus 802s (two channels per speaker) and the fifth channel powers my b and w center channel. My surround rears are powered by an old proton six channel amp that is bridged to three channels of which I use two for these two rear speakers.I have an Nad processor that controls the center and the rears and a Joule la-100 that runs the two mains. My center channel is cutting out while I watch movies. It does not cut out completely. You can feel the cone vibrate and hear very low level sound out of it. When I turn the volume control up and down on the NAD processor rapidly it will pop back on to full volume. After a short time it goes away again.Turning the volume up and down rapidly always makes the sound pop back on but it soon disappears again.The rears are not affected. What gives?