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)
the rad seems to be fine, there was loads of dirt and grime on the out side of the aircon rad which was restricting the air flow through the cooling rad but i cleaned that of and it wasn't that :(