On my router (MT7620A) I see quite a lot of dropped packets on the WAN-interface:
eth0.2    Link encap:Ethernet  HWaddr ....
          inet addr:192.168.15.2  Bcast:192.168.15.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:3250663 errors:0 dropped:183244 overruns:0 frame:0

I suspect, it might be correlated to these erros, I also see quite often:
[854522.540000] ralink_soc_eth 10100000.ethernet eth0: port 2 link down
[854524.500000] ralink_soc_eth 10100000.ethernet eth0: port 2 link up (100Mbps/Full duplex)

In logread, I also have quite a few other "strange" errors which might point to some
packet corruption.

To start digging into details:
How to figure out, which/why packets are dropped on the interface ?

And how to figure out, what might be reason of "link down" ?

Goto thinkbroadband.com
Setup a user account
Then create a Broadband Quality Monitor (BQM) on your WAN IP. You will need to create/update the monitor if your ISP assigns you dynamic IP
Then, it will graph packet loss and latency by hour by day. This will give you an indication whether you are a victim of congestion or something else.

Other fault areas could be:
* Modem/Router  CPU is maxing out.
* Bufferbloat in your ISP (traffic shaping/buffer too large) - check latency isnt over 200ms when maxing out downstream
*  VDSL upstream speed  maxed out.

Your ISP should be responsible for fixing certain types of probs but of course in terms of congestion I doubt they will because that would require them to increase their bandwidth (=investment). Another ISP might have less congestion around certain times but quite how you would know this prior to contracting with them ,I dont know.

(Last edited by ninjaef on 12 Feb 2018, 10:17)

Your WAN interface has a private IP address, so it looks like it is connected to an upstream router, is it?
Could it be a problem with the ethernet wire, or the upstream router?

Upstream router: yes.
Wiring problem: Very unlikely, as I have several of these routers, all with similar symptoms.
My first idea from old experience: Auto-negotiation might be faulty. But ethtool does not like the device type,
as a (supplier-provided, customized) CC-version running.

In this case, I would consider filing a bug about the driver for the ethernet chipset.