Combi boiler hot water fluctuation

Any suggestions on this? I have a 6-7 year old puma combi boiler that fluctuates the hot water between being extremly hot to cold (like running a cold tap) over a 2-3 minute cycle. I have tried turning down the thermostat on the boiler itself but this seems to make no difference. I have had 3 brit Gas guys come around and look at this yet they seem to think there is nothing wrong!? As a result of the fluctions having a shower is damn hard! This is not restricted to the shower as it also does this with both the bthroom and kitchen taps.

I've also noticed that if the central heating is running the water temperature becomes so hot you can't put your hand under it.

Thanks for any advice in advance

Temp sensors for CH and HW are separate. If the CH is on you should expect a hot slug of water from the tap when you first turn it on, especially if flow isn't great. Scale in the heat exchanger(both sides) and around the sensors can introduce delays in the temperature control which gives a problem like yours. Did BG not mention descaling/powerflushing?