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?
Burnt voice coil most likely - it needs a new cone.
Try cleaning the volume control on the NAD Processor.
Test the speaker somewhere else, that will tell
you if it's the speaker.