Always run the zetec's with the cap ON, they run very very hot, best way is to top to minimum run up to temp, then leave to cool at least 2 hours, as it cools the pressure created in the tank draws out the air in the system, then just top to min again and off you go.
I suspect the 2 dreaded letters as you do H+G!!, head off moment it seems, just look VERY carefully for a small fisure(crack) in the cyl head as this is very common with zetec so will need a skim.
tabetha