why does my swift flow boiler turn itself off?

i have a glow worm swift flow E boiler, when the radiators are heating up the boiler trips itself off after a while, the reset control light is on and the boiler fires back up after pressing the control reset switch, the pressure is 1.5 when cold and touches 2 bar when hot, there is a little red arrow on the pressure dial just past the 2 bar but i have not seen the needle reach this point. any ideas why it is tripping itself off ?