[b]Fluctuating Hot water - Combi, Runs hot then cold etc [/b

Have a Ravenheat WhiteStat 29T Combi. Approx 6 months old.
Since switching to winter mode HW & CH, have noticed that when running HW tap that get hot water for upto 20 secs then cold 5 - 10 secs then hot again & so on.
CH works fine.
Tried switching back to summer mode, ie only HW & still get same hot cold cycling effect but reduced - get lot more hot water, plenty for bath - so probably always been like this but not really noticed it.
With hot tap running constantly boiler comes on , goes off (fan still running) then fires up again after 5 - 10 secs & so on.
Tried changing water temp setting but make no real difference. A real problem when trying to get a good hot bath with CH on. Is this normal for Combis?

Any suggestions - thanks
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Approx 6 months old.

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It's still under guarantee! Get installer back first, then onto Ravenheat.

Thanks for the quick reply, will try Ravenheat as not sure who fitted it - only moved in 3 months ago, Was told that boiler was new (ish).