how can we fix our Vokera compact 29a lose presure ?

our vokera compact 29a boiler keeps losing pressure 3/4 times a week have had it service every year we had it fitted new oct 2012 we have cleaned and renewed the off and on taps on two rads and drained the water 3 time but still the same there is water driping on the pipe going out side and it comes on in the night by its self. if there is any thing I can do my self please can you tell me it in easy steps I am good at doing works but must be told in full simple details hoping you can help to help us to save as just paid for a service a few weeks ago thanks again