Hoping someone can help me with an issue Im having with my hot water. When ever the shower or bath are running upstairs in my house, the water starts off scalding hot but after a few minutes it loses its temperature, eventually running cold. However, if I wait a few minutes and run it again its back to being scalding again, or at least for a few minutes. The water downstairs never seems to cause a problem, and heat etc are fine too. The boiler is a Grant combi outdoor 90V3, which has always been regulary serviced. Hope someone can help?