It sure sounds like that circuit is not too well...it may be time to call an electrician. If it's good off an extension cord, then find out what breaker (in your main panel) that plug is connected to; I would assume it is a different circuit then the one the amp was originally connected from. It may be there are too many appliances/equipment, lights, etc. on one circuit and causing problems....like I said, I would call an electrician to further investigate....

I mentioned it somewhere here in the Saloon recently about a hum problem I had.....I had an old lamp with a very unique bulb in it; it was in a room in another part of the house....I noticed a hum in my amp for a few days; for the life of me, I could not figure where in my HT system it was happening, until one day, my girlfriend walks into the other room ( I was sitting in my HT room with nothing playing) shuts this light off.. AND THE HUM DISAPEARS...suffice it to say, I was overjoyed!!! (because amp hums can drive us nuts, LOL)...long story short, is this lamp holds sentimental value for us, but WHEN in HT mode, the lamp is turned off....