If you have copper wire, Chances are the you have a wire that is loosing contact as is gets warm from current flow. Quite often the receptacles and lights are on the same circuit (if the recptacles are 15amp)and two or three rooms are fed by the same circuit. Starting at the breaker make sure your wires are tight to the breaker and ground on the circiut feeding these rooms. Then follow your lead to the first item on the line (Light switch, receptacle, light, etc. chances are you won't get too far before finding a loose connection. Had one on my microwave that did this and it drove me nuts until I checked the receptacle and found a loose wire. If the recptacles or switches have been changed out at some time the new ones might have been backstabbed (wire pushed in from backside) . These are notorious for causing such problems. Sometimes you will pull them out to check them and one or both of the wires will slide right out !