The wiring in my house is 15 years old and stable. We have 5 GFCIs installed throughout. Recently early one morning, 3 GFCIs started tripping randomly. Nothing unusual was running. Not always the same GFCI. After about an hour of this, the problems went away and have not returned so far. My first suspect was utility brown out, surges or crews working in the area. The utility co. would not admit any outages or work nearby. Any other ideas to pursue ?