sounds airlocked, did you top up thru the header tank or the top filler plug (on top of thermostat), was the heater control set to hot??.. I had same thing on mine, cured it by jacking front up a little, or parking front end up a slope, topping up, running engine for 10-15 mins, let it go cold, check water level, and repeat, long winded, but worked, any air came to the front as it was highest point..
Having said that, a friend cured his problem of water blowing past the header cap, by fitting a new one (header cap) as the old one was letting water past at below operating pressure..