Hi, Can anyone give me some advice please?

I have a mira 415 running off a Worcester 24 combi, both are
approximately 5 yrs old and the boiler is well serviced by British Gas.

The problem I have is that the temperature of the water goes from
either freezing cold to scalding hot in a cycle of probably 30-50
seconds. No amount of adjustment of the control helps me. any ideas
anyone?

Also what would I need to keep the pressure/temperature constant if
someone flushed the toilet or used another tap?

any advice would be gratefully received. Thank you