Gloworm Swiftflow combi boiler

I have a very annoying problem with our 6 year old Gloworm Swiftflow combi boiler. When using the shower (just a bathtap mixer type one) the water gets very hot and then adding extra cold water to cool it down results in the water turning completely cold. The shower water temperature then oscillates between very hot and very cold.
When washing the dishes downstairs (the boiler is in upstairs spare room) the water becomes boiling hot and then goes cold and repeats this continuously.
I've had the installer come and look at it and he said it's fine and is just something that happens in summer so live with it!
Anyone got any ideas what the problem might be?
Thanks - Art
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Having a proper thermostatic mixer will certainly help. One is available from Screwfix for bath mounting. It does the bath and shower. Quite good.
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