My idea is this - the temperature of the incoming water versus the combination of the required temperature and flow rate of the outgoing water are not balanced against the heating power of the boiler.
If you adjust for a slower flow rate there may be time to heat the water as it passes through and that this problem will be more noticeable on colder days than warm ones.
You might also consider lowering the temperature for the hot water - there's a down-arrow button that does this.
But I only guess - I have never had the experience of a combination boiler.
Good Luck...
February 2016
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