Is it to do with your central heating being on at the same time as you are running hot water? I think the hot water temp with our boiler varies by a little bit if the water is circulating in the radiators at the same time. It is not indicative of a problem with the boiler.
do you mean it is a new boiler? Or has it changed since the cold weather came and the incoming water main was colder?
if new, it is quite likely more powerful than the old one. Modern boilers modulate the size of the flame to try to align power output to demand. Combi boilers have trouble maintaining a regular flow and temerature if other taps, WC or washing machine are drawing water at the same time.
run the shower into a bucket and see how many litres per minute it delivers - should be 10 or more. This will give a clue of the flow.
also run the kitchen taps into a bucket and measure litres per minute - will probably be more from the cold than the hot
do you find any of the radiators get warm, when CH is turned off but you run the hot taps?
Have problem with combi boiler where shower water began to vary between hotter and cooler than selected temp. It always seemed to be constant previously. Had boiler repairman out - he has measured temperature at various taps and shower and admits there is a varying range of 2-3 degrees with the shower - hoter and colder than tap position - say between 53 and 56 degrees. he says this is because the boiler is 'modulating' and is normal. I maintain my shower temperature was 'constant' ie - fixed and consistent before problem.
His diagnostic computer says boiler is fine - 'computer says no problem'
I think he can't be bothered to look further - but please tell me is your temperature fixed or variable.
he also maintains a variation of 2 degrees is not noticeable - I say it is.