Odd overheating

So my wife's Disco overheated the other day when one of the bleed bungs disintegrated, having replaced the bung it still overheats after 5 or so miles. I suspect the head gasket has gone and am sourcing a recon head.

However before I commit to all that work, it behaves it has classic symptoms of head gasket failure as the heater is cold when the temp gauge is off the scale, however if I turn right such as going round a bend or roundabout the heater will all of a sudden blast hot air and the temperature comes back to normal, then after about a mile or so it shoots up.

Any suggestions to explain this odd behaviour.

Air Lock??

I did wonder about an air lock but it is the way in which as soon as you turn right the temp drops it's almost like the steering wheel is operating a stop tap

Air lock in water.... Had the same with mine, so redone all coolant, new antifreeze and filled up 11 an a half litre's real slow, hoping to get all air out. seems to be fine now. good luck.