I've been advised in the past that *each* outlet socket must have a BB filter in it (not sure if applicable to unused sockets).
Also, that sounds like a very long cable run to the living room socket. Signal quality goes down with length and 25m of cabling must act like a radio antenna, to boot.
However, as you have already concluded, structural handicaps should impact your connection at *all* times of day. What you are getting is service throttling and/or borderline denial of service attack symptoms.
One of the things inhibiting me from changing to BT is that I don't know how much control they let you have over your router. Mine can email me reports of things its firewall has dealt with like port access attempts and DDoS attacks.