boiler playing up?

hiya, my alpha CB24 doea this thing every now and again. it switches off and the red error light flashes which in the manual syas its the overheat therkostat but even after replacing the heat thermostat witha new one it still does it. when it cuts of and you re=set it sometimes it wont come back on and you need to keep resetting till it does, or if it does come on it doesn'yt stay on long. then other times it'll run for week with no problems.