How can I fix my Baxi Bermuda 100 Combi Boiler?

My boiler keeps cutting out all the time. It fires ok but runs only for about 10 seconds, then it cuts out! this happens with both the hot water and the heating. i have pressed the reset button, but it makes no difference. it is really annoying now. The pressure gauge needle keeps dropping as well. I have checked for leaks, and there is no leak anywhere. please help