Sh*t sorry mate, thought you were joe pubic, not really sure what is wrong with your boiler, you didn't say what make it is, usually with tubular heat exchangers it's lack of flow through the boiler or over gased, wondering why you fitted 15/60 pump, is it a big house, using a 15/60 pump it may be pulling air down the vent pipe airating the water and causing problems, 22mm cold feed normally required, I would phone makers tech line to see if they have had similar enquiries.