afn passat cooling issues

need some info here heater keeps going hot and cold
and losing water if i screw on cap , ifi leave cap loose it only loses a bit
so
flushed rad ==to we get clean water
flushed engine to we get clean water
flushed heater matrix to we get clean water
refilled using bleed hole and raising reservoir 100mm up
refilled and its just the same....
cant seem to get bottom rad hose hot even when engine is at operating temp
checked thermostat and it opens ...
viscous fan runs
losing what hair i have left not much to go
anyone

The words I notice are : "is losing water if I screw on cap"
So when the expansion tank cap is on it pressures it out? That's not good, I assume it was not doing this previously, or you have combustion pressure getting in coolant. You can test for this, get test papers like a litmus strip from an auto or truck parts supplier. It tests for carbonates in the coolant from combustion pressure. If that isn't a thing, it is probably as simple as the heater core being full of air, and expanding when hot. Heater going hot and cold can be an air in the heater core problem. Can usually hear it gurgle under the dash when you first start the engine.

well it turned out to be head gasket ,,,so decided to break her thanks for input