i have the exact same problem with my car, i can't find the couse ether, i've changed the rad cap and didn't work, the last time i went out i came back and parked up and as soon as i turned the ignition off about 2 ltrs of water excaped out the expantion tank even though the temp gauge was at normall :( it's just sat there since, if i don't find the problem soon i'm going to scrap her which i really don't want to do
i had the same problem with a gt4 that i had, after the head gasket blew (coz i kept driving it like this) i found the problem to be a blockage in the cooling system (turbo water feed)