baxi combi 105e hot water fault?

I have a problem with a Baxi combi 105 e which is that hot water does not work on upstairs taps if central heating is on or temperature lights show more than 20. Plumbers took apart the diverter and found no fault with it. Hot water works if central heating has not been on, and central heating works fine. Also, the temperature of the hw does not regulate when you change the temperature dial on the console. Help!