Baxi 105e Combi - Hot then cold water cycle?

Hi, I have a BAXI 105e Combi boiler in the house I have just brought, it was installed in 2006. I have noticed a problem with the hot water upstairs, with the shower and bathroom taps. It runs hot for abit then freezing cold, then back to hot in a constant cycle. However, there doesnt seem to be a problem with the taps downstairs, in the kitchen. The boiler is located downstairs in the garage. I have no Idea of the state of insulation of the pipes, but this wouldn’t explain for the sudden drop in temperature? Also the central heating works fine and the problem in there with or without the central heating on. Any suggestions? Many Thanks.
Lawrence