Well I have been through this with mine I had resigned to the fact I had a blown headgasket after I thought I bled it properly. I ended up taking it to stechnic at Cannington who picked I had the wrong pressure cap on had thermostat changed and a genuine suby thermostat but looked very old and bled it properly with a special funnel they use.

Now it has been about 4 days with a bit of on boost action it is has been fine so fingers crossed if I had gone on boost before she would overflow the coolant. The thing with mine is you drove it off boost it was fine.

Good luck Mark

Oh also they did the tk test for the headgasket on mine showed nothing...