@Marvin_S Thought I'd just follow this up. It's definitely the TD-W9960 at fault, having replaced it with a DrayTech in bridge mode which does not exhibit the fault. Putting the TD-W9960 back in...

@Marvin_S many thanks for the reply, and apologies for the delayed reply. The DSL light stays lit at all times, and the Mikrotik firewall/router on my side of the TD-W9960 never shows the PPPoE link...

TD-W9960(EU) v1.2 (setup in Bridge mode, wireless disabled) stops forwarding packets for around 5 seconds every few minutes. It might do this twice in one minute, and then not for another ten...