trouble bleeding the cooling system?

you gave advice on T belt tensioner on my focus 1.8 tdci. I have another ?, i had trouble bleeding the cooling system of air after fitting a new WP. 400 miles later the engine runs fine but im not convinced that the engine is cooling properly. the rad intermittantly feels cold to the touch (hot cold hot cold). it reaches temp, engine runs fine, hot air through blower, fan cuts in. but temp gauge reacts differently to what it did before (does not overheat), do you think this is an air lock in rad that will eventualy clear itself.