yep do all the basics like cleaning oil coolers and radiator. also in between them.
Does the upper radiator hose get hot? if not I would put in a new thermostat.
Are you sure your not losing any out of the overflow tube?
if you are losing it out of the overflow then that's good news (in a way). I would put on a new radiator cap.
if not, that could point to it going into the engine internally were it isn't suppose to. How does the oil look? any white smoke (steam) from the exhaust?