Gaggia Titanium - keeps going to ventilate after steaming for a short time.

The machine had been stored for a while after a fault had developed - water was coming out of the wand as well as through the coffee side. The micro switch, steam valve and silicon tube have all been replaced, the brew unit has been cleaned and re-greased. The machine has worked for a couple of weeks but now keeps going into ventilate every time the steam is used. When I got rid of the ventilate message I de-scaled the machine four times over, the last cycle I switched the machine off and left the solution in the machine overnight before continuing the next day. As soon as steam was selected the pump would work getting quieter then ventilate would be displayed. I've checked the flow meter, it's clean, I've blown into the part where the water pipe connects, the turbine can be heard whizzing round, there is water in the pipes. There was a little water where the thin pipe with the brass ring on goes into the steam valve, I swapped the o ring for the old one from the original steam valve. The steam worked for longer than usual, almost steaming a jug of milk to temperature before the sound of the pump getting quieter then the dreaded Ventilate message appeared.