What is the problem?

Baxi 105e instant, pressure in hot tap in kitchen very strong, but no hot water, upstairs you have to turn on wash basin tap full, pressure decreases after a few seconds, water gets hot, you can open tap in kitchen to give hot water, as soon as you turn tap off boiler will not fire unless you use taps upstairs, heating works ok, if preset is turned on full boiler fires and you will get hot water from kitchen tap even if preset is switched off.