I have the same boiler which I installed about 5 years ago.
It has similar fault as yours.
start from cold flame comes on, goes out comes on, goes out, then fires
normally, for a cycle, then same again, until it is warmed up a bit then its
okay?
I have an external bolt on stat on the pipe.
I'll try the by pass and see if it fixes mine too.
I have tried moving the pots for soft start,cycle delay, etc, no change

Keith wrote:

My Ariston Eurocombi Boiler has developed an intermittent fault, where
it shuts down (flame goes out) for a second or so, then fires back up
again, whether supplying hot water or the radiators. This repeats
constantly, every 20 seconds or so, whatever temperature the HW or RAD
temp controls are set to.

Fault finding suggests an intermittent fault in the boiler output limit
thermostat, and temporarily bypassing the 'stat restores normal
operation, but is obviously not a safe way to operate the boiler.

Looking at the boiler innards, the design appears to have been modified
since the manual was written, in that there's a bracket on the output
pipe for the 'stat, but tracing the wiring diagram the 'stat now appears
to be integrated in the DHW temperature control, 'ganged' with the DHW
'stat.

Replacing the DHW control would appear to be a high cost, labour
intensive task, involving draining-down and dismantling, but simply
bolting on a previous-model pipe-mounted 'stat and re-routing the
existing wiring would see the job done in a few minutes.

Can anyone see a problem with that idea?
Would I need to use a thermal transfer gel/paste when fixing the 'stat?
The DHW 'stat appears to be working perfectly, switching hi/lo at the
set temperature, but is there a common failure mode which could see the
DHW 'stat fail soon, making it worth replacing the control now?

Thanks in advance
--
Keith
Note NO....SPAM measures, the fix should be obvious