a friends s2 is having serious overheating probs, we cant work out why, even with both kenlowes on it still gets right up near the red and the water in the header tank bubbles like mad, when the cap is taken off shit loads of air comes out


things we have changed so far;


rad
thermostat
coolant hoses
header tank
pressure cap
Y peice (that goes into the heater matrix) - had a small split in it


while running cool the car drives spot on, but boils up very very quickly... there isnt any sludge in the header tank or in the rocker cover - so the head gasket should be fine?

there doesnt seem to be any water leaks from anywhere either


any ideas???