Boiler overheating and tripping?

Hello,
I have a alpha combi boiler. Since yesterday, when i try to start the boiler, the flame starts to die down in 5 seconds, there is lot of pattering noise, the flame goes out completely, the pressure moves back and forth for 2 seconds and the overheat thermostat goes red and the boiler goes off. If I press the reset button, the whole above cycle repeats itself. Pls help.