a head gasket will not always show loss of power or smoking. infact it can sometimes increase power slightly (starts leaking very slightly into cylinder(s) and causes a slight power gain from the introduction of water to combustion mixture). best bet is to do as the others have stated and check the obvious things. next do a pressure check on the radiator to confirm that its not losing pressure through a headgasket or minute leak. if your not losing any cooling in the reservoir and/or radiator i would suspect a sticking thermostat.