BAXI 105E HOTWATER PROBLEMS?

My Baxi 105e gives occasional hotwater, I changed the diaphragm and it worked well for a week. It stopped working again I dismantled ithe housing again and cleaned it up a bit but every now and again it doesn't give hot water unless you tap the front bottom left of the boiler casing. Could it be i need a new housing for the diapragm or is it something else? Many thanks for your help