Thanks for the reply.
More often than not, it does seem to happen during a longer burn cycle. From my many observations the pilot goes out (along with the burner) during the burn cycle without, not after.
I’ve never seen any signs of condensation or heard a sizzle sound. There’s absolutely no rust on the burner or in the combustion chamber–I assume there would be some rust if this was a chronic problem.
Short of catching it in the act, are there any other tell-tale signs of condensation?