What about wireless interference? The speaker wires or components in the amps might be picking up something external in the air. I had a problem with my HDTV pixelating intermittently. Turns out it was a cellphone that was on a frequency that was a close frequency to particular channels and the cable box unit was picking it up. Perhaps some device in your house is not shielded enough, malfunctioning, or too close like microwave oven, cellphone, cordless phone system, a wireless router, satellite dish, near the path of a business microwave communication link.
Another idea is to have your electric main on the outside of the house checked by the electric company. I believe they will come out for free to check this as the core in the box is their equipment. These cores can go bad and if they see the specs are out of normal they will replace it. They will probably put a new one in anyway since they are there already. If they do their job correctly they will also check that you are getting two solid, clean 120 (x2 =240)connections to the house from the pole as part of the call.
Another idea is to have your electric main on the outside of the house checked by the electric company. I believe they will come out for free to check this as the core in the box is their equipment. These cores can go bad and if they see the specs are out of normal they will replace it. They will probably put a new one in anyway since they are there already. If they do their job correctly they will also check that you are getting two solid, clean 120 (x2 =240)connections to the house from the pole as part of the call.