What is wrong with my vokera mynute 28/96 SE boiler?

My boiler either cuts out or fails to start. It is losing pressure and I have to keep adding water to keep it from falling below 0.5 bar. There are no obvious leaks. The boiler hums but the pump and fan fail to kick in. The boiler is approximately 10 years old and is the Mynute 28/96 SE. Sometimes it starts and works OK then cuts out after a couple of hours.