Variable water temperature puzzle – please help!

Hi, I have just had a new Main multipoint bf boiler fitted to supply hot water only. It seems to work perfectly and there is hot water in all the taps with one exception – the tap in the kitchen which is furthest from the boiler starts cold as expected though soon runs hot but after a few minutes runs cold again, then goes hot again a little while later and so on. The stream of water doesn’t alter in quantity, only in temperature. Is this a problem with the tap, the boiler or pipe length or something else? Our plumber is baffled, so any suggestions would be welcome! Many thanks 🙂