A very simple test for HGF (which has been already said is uncommon on the V6) is take her out for a 10 mile thrash at motor way speeds - park her up and next morning undo the expansion tank cap - if you get an audible escape of gas then it points to HGF as there should be no pressure in a cold system - that points to exhaust gases in the coolant pipes.
Also are you seeing mayonnaise under your oil filler cap or on your dipstick - if you have HGF you would often see emulsified oil deposits at these places - aka "brown mayo".
