Erratic 156 v6 overheating

[cumbrian bob]

Erratic over heating 156 2.5 v6.
Hi can anyone please offer advice on my mates v6,we've flushed out the entire system twice including matrix,bled it to death,put new thermostat in,and it still over heats now and then.When you start it up it blows out cold air even though the top hose is hot and the bottom is cold,but once you turn the heat control knob it blows hot air and the engine starts to overheat,any help would be massively appreciated.