you left the cap of a little bit to long lol. just fill it up (with hot water other 3wise you mite crack some thing (done my header tank this way )

leave it running till the temperature gauge is moving, water should be coming fluently out of the swirl mpot pipe.then put the cap back on.

failing this, your losing water compresion (the cap raises the pressure on the water system to 18 psi to raise the boiling point) the could be a split core plud or at the worse a split or cracked head.
is the fan comming on,
and did you replace that thermostat?