Intermittent hot water problem?

I have a Vokera Maxin 28e combi boiler and it has started only providing intermittent hot water. This makes itself known mostly during a shower when there's hot water for a couple of minutes then cold for a couple, then hot, then cold, etc.

At the boiler, while this is happening, the DHW diverter pressure sensor appears OK because the microswitch is closed during all this.

I took the DHW heat exchanger out and gave it a clean but it seemed to be already pretty good - there were only a few little bits of stuff that washed out.

At the boiler basically the gas comes on, once the initial demand for hot water has caused the microswitch to close, and that's OK. The water does get pretty hot, maybe too hot, and then the gas goes off. After a while the gas will re-ignite but the water has run cold by this time.

Any idea what it might be?