Yes, exactly the same thing happened here, just before Christmas.
All 4 ethernet ports now very unreliable, lots of packet loss apparently.
At random, on any port, I am unable to even get a ping response from
the router, and the admin web interface gives "internal timeout" errors.
The WIFI connections work just fine, using DHCP as normal. Weird.
Right now, things seem modestly better. I know the clients are clean,
but otherwise I would think the router is getting a DDOS attack from
somewhere, or my ISP went bad, but the WIFI connections stay fine.