Hot water prob - Vaillant Turbomax Plus?

For about the last 6 weeks or so our hot water temp has been very unstable.

The boiler fires and heats water for about 10s then stops heating, the temp soon starts to drop fairly rapidly giving luke warm water for about 20s then the boiler fires again to heat it back up, then... repeat.

This cycle is constant when running hot water and is particularly noticeable in the shower (of course) resulting in a mainly lukewarm shower with regular but short peaks of decent hot water.

Prior to this we had a similar problem, which eventually resulted in no hot water availability at all. This appeared to have been caused by a radiator leak and very low boiler water pressure, and was apparently cured by fitting a new rad and re-pressurising the system.

The pressure is now fine and stable at approx 1.5bar when the heating is off. The heating appears to work fine.

Apparently common problems that may cause this include Diverter Valve failure, Air pressure switch failure and thermistor failure. I've also read people mention thermocouple issues causing things like this?

I did have a quick play with the boiler's diagnostic mode and the values for the DV and APS appear to be ok and working, but I don't really have the experience to be completely sure of that. One thing that did seem a bit odd was that the diagnostic value for warmstart said 1 (which apparently means on) when I'm certain I've switched that feature off.

Any help, advice etc much appreciated (so we can have decent hot showers again Even if it turns out to be something I can't fix myself, any help in identifying the problem before calling a pro out to help would be really great.

The boiler is a Vaillant Turbomax Plus, about 5 or 6 years old.

Cheers
Martin