59 1.8TDCI 120K spilling coolant from tank at high revs

Hi guys, the wife's Galaxy has started spewing coolant from expansion tank once warm, and seemingly only when pushed to around 3000 rpm. I have run the car at low revs around town for up to an hour and very little or no coolant loss, though you can hear the cap venting after a long run. I've changed expansion tank and cap, new thermostat. Water pump changed 12 months ago when I had the wet and dry belts changed. Runs great, plenty of power, no excess smoke or steam out the back. Never overheats, needle stays around 90 once its heated up. Had it on laptop and both temp readings come back at 91, Also forced fans on to confirm operation . Heater blows hot in the cabin as expected, i can see coolant returning to the expansion tank from the small pipe from the thermostat housing. Compression test showed all cylinders comparable. Sniffer test (performed with hot and cold engine) showed no colour change in test liquid after plenty of bubbles passing through it, also confirmed blue test liquid was good by putting on exhaust pipe. No water in oil. Does anyone have any experience of similar issues? I'm thinking small head gasket leak thats only showing up under high load, but the more I read up,  I'm lead to believe these lynx engines are normally bulletproof. Any help greatly appreciated.

Cheers

Dave