Sounds like could be a couple of things. First (and probably most unlikely) is that it's being blown out by high /gusty wind. I once had a balanced flue boiler with short balanced flue through side wall and this used to happen often, but if it's a Baxi style, non-balanced, with vertical chimney-type flue this is very unlikely.
Much more likely is the thermocouple is getting tired. Cheap option would be to try replacing the thermocouple and see if that cures it. Not sure what this would be, probably around 5 to 10 quid but worth a try before calling the pros in, if you're happy working on the boiler?
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