Revived this one as still having an issue

I have done / replaced the same items as AKIS. Hoping to get hold of him, but he has left the forum (probably due to this fault).
the HVAC systen works fine, the heater will work quickly from cold - However as soon as you reach 2000 RPM, it instantly goes cold (regardless if you are moving or parked). As soon as the revs drop the heat instantly comes back.

I have traced this to the control valves, as the metal heater pipes go cold & warm again. Obviously they are receiving a signal to close then reopen. I have also changed the control panel (New & another second hand one) but still the same.

I've noticed that the blend flaps will not always change manually. They may work on inital startup, or by switching the ignition off & back on again.

I have been to JLR & Indy's without a clue. I feel that the control valves are getting a signal to shut at the higher revs, but from where?. Temped to change the BCM, but feel that this is a bit OTT.
Question if this is water pump cavitation, as it cools & heats up again so quickly - Really off with this now!