These "WAN Connection: Ethernet link up"s come regularly, but if I have no heavy downloads they disappear. Who would you guess is more likely culprit: the router or powerline adapters? Have no way to replace adapters to see if anything is changed. And yes, 374.40 on AC66.
Highly unlikely to be the router (unless the Ethernet port were physically detective). Ethernet link down/up messages refer to the physical link between the WAN interface and the modem (or in your case, the powerline adapter).
Start by replacing the Ethernet cable itself between the adapter and the router.