If this power loss and flickering are only on one circuit, chances are that there are problems with a loose connection in one of the boxes/outlets on that circuit. Tighten & inspect all your buss connections first. If the problem persists, I would check that gfci next. Lights flickering and a buzzing sound from the gfci tells me there is a loose connection there or that the gfci is bad. If the connection is poor, it tends to fail after a period of time when used because of heat generated.