I have a FERROLI COMBI 77 FF. Sometimes it works perfectly, then all of a sudden NOTHING. I can hear the system trying to fire up, like a whir whir sound. NOTHING for a while then it kicks in again. No heat or hot water for a day & then boiling water for a day. My head is done in. Any ideas? Please bear in mind I hope it is just something simple that I can fix myself. Thanks.