I don't know if they checked it, I will ask. But when this was happening to your car did your car get back to normal after some time? Because when this happens to my car, after 30min - 1h max my car gets back to normal and drives normal again with no power loss or anything.I'm sure the garage has already checked, but you sure the Head Gasket hasn't gone?
When it happend on my Fiat Punto (Mk2) the engine was only firing on one cylinder and power was RIGHT down. Engine shook and constantly died (whilst I stupidly limped it to a Garage). All of this happend after my water pump when pop and the hose split. Obv checks are cream in the oil filler cap and/or oil in the rad.