Gremlins in an Ariston Microgenus

Any ideas ?
My Micrigenus 23Mffi fires up, when on either Hot Water or C/Heat mode, then dies after 10 secs.
( the red LED comes on)

Water pressure fine.
Water pump activates.
Fan activates.
Gas fires up.
Can see the ignition spark ' asparking'
The Flame safety device in the flame appears fine.
All's well... for 10 seconds at a a time. But it shuts down totally.

Overheat sensor repaced.
Water temp pobe replaced.
( the easy bits first ! )
Checked all visible terminals.

Just to add insult to injury, the gremlin will go away, and the boiler works perfectly.... C/H or H/Water.
Until...

Three weeks later, same thing .

No good me calling out an engineer to a working boiler, so...
Any ideas ?
Bill

If I remember rightly sometimes the flue restrictor can cause problems in this boiler. Make sure the right ones installed or remove it.

Thanks.
I'll check that. Meanwhile, the gremlin has left ....
Next time . I guess it will be a call- out to run thro the system... I'm just trying to see if there is a simple cure. (The sensors were only £3 50 )
Keep well, Bill