EJ8 Problem

Hi all my ej8 is playing up a bit ive got an idea its a head gasket but could be alot more.
When it gets up to running temp it has trouble starting sorta coughs a bit then after continuosly winding her over she will start and wont hold any water at all you fill the radiator and it just pressurises and pushes it back out through the overflow bottle(which is a classic sign of a head gasket). Also when u first wiond her over she starts fine and i have about 2 minutes of full power then when the temp comes up she just dies. So just wondering if anyone has got any ideas on wat else it could be if anything.
Cheers
Cooper

Still has the sam problem any ideas?

have you checked the thermostat

Yes pulled it out and took the middle part out of it.