speakers cut in and out

I have two NHT VT2 front speakers and three NHT VS2 for center and surrond. At times one or both of the VT2 or one or both of the rear VS2 speakers cut in and out. It acts like a loose wire or bad connection. I tried it with two different processors and from different audio sources but still cuts in and out at times.I have a Parasound three channel amp for the front an a Prasound two channel amp for the rear. I checked and cleaned all speaker and imput and output connections but did not help. It seems to me the problem is in in the speakers.Has anyone had this problem or know what it could be. Your help would be appreciated!!!
if it's happening on all your speakers with different amps and using different sources then it strongly suggests a significant A/C power problem (or there are gremlins in your house).

I suggest you replace your surge protector or your power strip (if you use either....or maybe the wall outlet/circuit breaker.
Everthing is pluged into a Panamax surge suppressor. I will try pluging them into the wall outlet direct and different outlets. Thanks!!