2003 BA XR8 Dumping water
 

problem started december 2008 while on my way to wauchope (NSW, AUS) the vehicle dumped the water out of the header tank, it was towed into wauchope where it under took pressure testing and so forth and no problem has been found...we've found out that it is pressurising for some reason and blowing it up around the cap on the header tank...the cap has been replaced along with the thermostat and top hose bottom hose etc. and the radiator has been cleaned out...in april drove 1600kms (1000 miles) and it was fine, the next day we drove maybe 20 kms around town and when we got back to where we were staying it dumped it again...many thanks in advance it is starting to peeve me right off

update: it does not get hot until there is insufficient water in it (as it would)