My STi was doing the same thing. Sometimes under boost conditions (from mild to max boost) my temp needle would rise above the standard mark. For me, it turned out to be a head gasket and blow ring seals. I was pushing combustion gasses into the coolant which would gather at the top of the system which was causing the temp needle to show higher than normal temps (i.e. reading the temp of the gasses, not the coolant). If it happens again check your expansion tank. If the coolant level is way above normal then most likely you have a similar problem with gasses getting into the coolant.