I have a Baxi System 30/65 Boiler which is about 2 years old exactly.

In the past few days it has started mis-behaving.
It will run perfectly for about 1 to 2 hours, then it shutsdown and
the Flame light (first red LED) flashes on and off.

If I turn the dial to Reset and back it will run again for another 1
or 2 hours.

I had a look in the service manual and it says check condition/
position of flame sensor, which I did. Condition wise it looks OK, not
pitted or badly burned or anything. Can't comment on the position, it
appears to have the tip in the flame while it is running, but I don't
know what position it is supposed to be in. Can't imagine it has
moved.

What could be causing this? Any ideas?