It is possible that the cause is FuturePLC's service provider is throttling traffic going out of this website.
I would also like to point out that the dates that these incidents are occurring are usually dates when large amounts of people go on this internet. I am not a doomsayer or a devils advocate here, but if my hunch is correct that the service provider is throttling traffic going out of this website (after all net-neutrality in the US had its laws rewritten during Trump era), 504 may be seen again during New Years.