Expansion tank coolant level up & down

Disco 2 V8 4.0. Expansion tank was intermittently pressurising A LOT. On a couple of occasions it lost nearly all the water & overheated. No sign of where the water is going - I think most came from the expansion tank overflow & was vaporised on the exhaust. I have tried various fixes and they seemed to work but the problem returned after some time (like OK for 500 miles then suddenly happened again). This may be because a previous owner has used some sort of gold flecky sealant - I started to see this in the water, this has now all been purged (I hope). Eventually I got to the point were it was less intermittent and regularly lost about 1ltr per 10 miles, I could therefore know that a fix had worked. So my last fix was to install a new radiator, and this seems at last to have cured the loss although I cannot seem to bleed out the air & still have some gurgling from the dash. But.... the level still goes up & down and inch or two. Sometimes it stays level, I cant seem to reproduce the conditions. Also lots of gurgling (heater matrix I expect) but this has not accompanied problems in the past.
Engine - runs perfectly, instant start every time, never misfires, plugs all look normal, just a little soot as expected. Gas analyser shows no leaks. Pressure testing shows all cylinders OK. No water in oil or contaminations at all.
replaced: thermostat, water pump, header tank, pressure cap, radiator, most hoses. system refilled as per manual to try to eliminate trapped air - but I suspect this is the culprit.
What do you all think..

Thanks.