sounds like something is blocking the water, is the water being pumped round do you know ( waterpump broken? ) like most rst my fez ran really hot, i changed the thermostat and put a bigger std rad in, and a little water wetter. and well was never a prob again.
i would say fit a std header tank, not a fan of metal ones, I THINK( maybe wrong) that the metal with keep the water hotter plus you can see whats going on with the water better.