What's wrong and how can I fix my boiler - Baxi Bahama 100?

My above mentioned boiler has become very frustrating indeed! It will rarely stay on for a sustained period, cutting off all the time. It will either need the reset button pushing at regular intervals or it just won't fire up. Also most of the time the hot water won't work if you turn it on two times in quick succession. I don't know if this is linked to the problem but the pressure seems to be decreasing slightly in the system but there is no evidence of any leaks on the radiators? Any ideas and how much will it cost??!