66 plate transit luton

[Ged6362]

we brought the van from a garage, seemed to run great,
first long journey & over heated, temperature went into red & lost power, regained power when i took foot off accelerator, has happened a few times since then & we have to open expansion tank to release pressure, even when left over night if i open the expansion tank the next morning its still under pressure.
Can anyone help i have put into several garages & taken to Ford & no-one can give me an answer on what is causing this to happen,
Ford think it is possibly the head gasket, have had thermostat housing replaced from another garage & another garage has suggested new water pump.
Any idea's please ?