Disco 2 Hot running V8

Hi All, I'm looking for advice, I have a v8 Discovery 2 that is showing signs of overheating. Here's what I've done so far. Presure tested and only issue found was a leaking radiator. Changed Rad, running hot, changed thermostat refilled with coolant and bled. Ran up found split hose, changed that and header tank cap. What's now happening is that the temp gauge is at normal while sat idling no issues there took it on a 2 mile road test and temp gauge moved up still in the white but moved up to the top all the same. Stopped at the side of the road and the temp came down moved off and the temp gauge moved up slowly. now here's the weird bit. Parks up for a few minutes engine off and temp is almost normal, now nursed it home and the temp is normal. Coolant has been blown out the header tank. What's causing this as it looks like back pressure is building in the block. It had a rebuild 20 months ago with heads off skimmed and new gaskets etc, new water pump and radiator. Any suggestions welcome

Looks like it another head gasket is required, blowing coolent out of the header tank is a symptom of an iffy gasket.