Any joy finding the leak? Bottom radiator hose getting as hot as the top one? If not thermostat could be duff.
Plastic tops on radiators are known to crack and leak under pressure so check that too, could be along the front side so not obvious as coolant will leak down between the rad and the air con matrix. Even less visible if you have one of the aluminium slam panel cooling things fitted like this:
A lot of the newage cars are getting to the point where the original radiators are pretty shot now anyway...
Tried another header tank cap in case your current one is duff? Although I'd expect it to be obvious if the coolant was leaking out of here, it might not be holding the system at the correct pressure.