A lot of showers come with filters, have you any fitted. Try turning shower on and open a hot tap slightly, if there's not enough water coming through the boiler it could overheat, this will keep turning it off, then as it cools it will fire up again.
P.S. Changed loads of plates on duotecs recently, take it out and check for blockages.
could well be a plate exc issue ? had it a couple times on showers only / bar type thermo showers ? after they had been running for a couple of minutes
try turning HW temp down on combi ? see if it makes any difference ?
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