If you have already changed the GFI, I would look at the other end of the wire feeding that circuit, or a circuit protected by that GFI, looking for a loose connection. Sometimes a loose connection will cause just enough current leak to trip the GFI. If the wires are just pushed in to the back of the outlets, strip the wires and bolt them to the sides. Intermittent electrical problems are the toughest to find. Make sure all power is off before you do any repairs and contact an electrician if you do not feel comfortable messing with electrical circuits.