This is usually caused by having a loose connection somewhere in the circuit. Unfortunately, it could be in the fan box, the switch box, or anywhere along the circuit in any switch or receptacle box. Usually what happens is a wire being put under a wire nut isn't fully inserted and just sets against the other wires until there is a load on it for awhile and arcing starts to occur. The other situation that often comes up is having a fan on a receptacle/lighting circuit where the wiring to the switches or receptacles is connected through the stab in on the receptacles instead of a binding terminal screw. A long term load on these tiny connection points will often cause arcing and weakening of the springs that keep the contact tab in place.
Start at the most obvious place- the fan- and work your way back. Find which receptacles and light switches are on that circuit by turning off circuits until your fan goes off, then check to see which receptacles are dead...
Bob
