Boiler keeps cutting out

[Sydneytoo]

Hi, I have a very old Ideal Mexico Super RS65 boiler that until this week has remained very reliable. After it has been running for around 5 to 10 minutes there is a click from the gas valve and both the burners and the pilot go out. I can reignite the pilot immediately using the spark igniter and the burners will then reignite for another 5 to 10 minutes.
The thermocouple has been replaced but the problem continues. The pilot remains lit when the heating is off so I think the thermocouple is not to blame.
I tried turning the boiler thermostat down while the boiler was running and the burners went off leaving the pilot lit as expected. When I turned the thermostat up again the burners reignited, so I think the boiler thermostat is OK.
I also tried turning the pump speed up but the problem continued. The radiators and water heat up quite quickly while the boiler is working so I don't think the problem is the result of flow restriction.

The boiler is relatively simple with very few components. There is no down draught thermostat. So I'm a bit stumped.

Does anyone have any ideas?
Thanks

 

[45yearsagasman]

Did a pro replace the thermocouple, or did you do the job?

 

[Sydneytoo]

An engineer replaced the thermocouple as he thought it was just a problem with the pilot light going out. It seemed to have fixed the problem (for 24hours) but today it became clear that the boiler was actually cutting out rather than not starting up. I'm now wondering if we might need to bite the bullet and get a new boiler or if it is simply a symptom of a faulty gas valve.

 

[The831Bunny]

Why not get the RGI back to check? Intermittent pilot outage can be attributed to quite a few things, but the end result is always the same, a fail safe shut off. I'm sure he would want to be the first to know you are still having trouble.

 

[Sydneytoo]

He can't make it until the weekend so I wanted to try to diagnose as much as possible beforehand. One thing I have now tried was to manually rotate the three way valve almost fully to HW (to heat the hot water system as much as possible) and now the boiler seems to keep going. It cuts in and out normally when it reaches temperature and the pilot stays lit. The radiators are still getting fairly hot as well.