That is caused because the typical circuit is around 1,500 watts. One iron can be that much alone when it is heating. So, whatever else is on the same circuit, say your lights, then expect to see "brown outs" when the power is pulling more than the circuit was designed for. Try to find out if you've got a different circuit in the same room. I was lucky, DH brought in two additional circuits to my sewing room, and one serves only the iron.