I have one circuit in my house that has a 10 volt drop at 30 second intervals. Literally 30 seconds on the dot. Some other circuits in the house have a 2 volt drop every 30 seconds and yet others have no voltage drops at all. I have tried at the panel turning off every circuit or device with a motor attached and the drop still occurs. Also any resistance type heater etc have been turned off and the drop still occurs. The original wiring of the house, circa 1989, has 4 runs of 12/2 coming into and leaving a duplex outlet in the suspect circuit. What could be causing the voltage drop?