Ford Focus overheating

Hi, I have recently purchased a ford focus zetec. Very soon after getting it, it started to overheat, the mechanic I had check it over, said it was a valve and although it was showing (too hot) it wasn't really overheating. We changed the valve but this didn't solve the problem. I took it to another garage, they told me nothing wrong and to keep driving it, problem was it cut out when the temp went into the red. Took it to another garage. After a changed water pipe, new thermostat, head gasket and last of all ecu unit, the garage decided to take out the thermostat to see how it was. After driving approx 25 miles( Not Continuous) the engine overheated yet again and I had a leave it at the roadside. After a few hrs of calming down I went back to the car and it was empty of water. I filled the radiator up and took it home. I am now at a loss of what do do next as nobody seems to be able to find the problem.