Is an Ariston microgenus and still happens when everything off like washer/dishwasher etc.
Have found that if we run the cold tap in basin at same time then hot water in the shower is more stable.
I have a downstairs combi boiler and upstairs shower. Recently the water temperature in the shower keeps changing from hot to cold to hot constantly. Also this happens when running the hot taps upstairs but downstairs ones are ok. Pressure gauge on boiler seems ok and there are no visible leaks...