Boiler short cycling

Real diy question.

My son's boiler is short cycling when it never used to.

WB Greenstar 30i combi, magnetic filter, just 3 years old, serviced annually. 6 rads, towel rail, 3 storey house, inhibitor in since install.

It used to be just fine but just before the big cold snap he complained it "wasn't right".

From cold it warms up the rads but as the water gets hotter it cannot control the temp. e.g. demand was 67C (yes I know that's hot!) and it warmed up the water. It sat saying 68.5C for some time and then as rads got hotter the temp rose slowly and slowly till it got to the over temp and the gas valve closed. The temp display dropped quite fast at first then slowly dropped whilst the pump stayed running to about 51C. Then it would light and the temperature rises fast. I timed it and it got from

50 to 71 in 1m45s.

Pressure is sat at 1.5bar on the gauge and didn't seem to rise from when it was first lit.

There's one rad down stairs that doesn't get very hot. Other rads are reported as being hot at top and bottom.

Is this indicative of needing a flush?

Any other suggestions?

That sounds like an oversized boiler, unless all the rads are huge, and it has no insulation.

Turn the flow temperature down to 45 or 50, it could be the system is not big enough to handle the heat given out. It may have worked before, or it may have some setting changed, we dont know. Also, see if the pump has had its speed changed, or is it variable speed set by the boiler? Worst case is a partially blocked heat exchanger, which is so expensive to replace, it may mean new boiler time. One rad not getting hot coudl mean sludge in that rad, or in the lowest level pipes. What is the filter like? Clean? Inhibitor in date? But first, turn down the flow temp. to less than 50 degrees,and see how that works.