UKPassats

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