How to determine why kernel is dropping packets

[Alex]

Hi,

I'm having a problem trying to figure out why the kernel is dropping
packets on any machine connected to my cable modem and I don't know
why. I've tried posting to fedora and network lists, but haven't had
any luck, and hoped someone could help me here.

I have a fedora28 system with 4.16.12-300.fc28.x86_64 on a Xeon
E5-1650 v3 @ 3.50GHz system, and during regular activity, it's
dropping packets:

# ifconfig br0
br0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 68.195.193.42  netmask 255.255.255.248  broadcast 68.195.193.47
        inet6 fe80::6c26:fdff:fe03:7f7e  prefixlen 64  scopeid 0x20<link>
        inet6 ::ec4:7aff:fea9:18de  prefixlen 64  scopeid 0x0<global>
        ether 0c:c4:7a:a9:18:de  txqueuelen 500  (Ethernet)
        RX packets 202664225  bytes 255532394512 (237.9 GiB)
        RX errors 0  dropped 391461  overruns 0  frame 0
        TX packets 130138618  bytes 91417518889 (85.1 GiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

This occurs on the other similar system directly connected to this
Optonline cable modem as well, with a different kernel, so I don't
even know if it's the kernel itself. Perhaps it's a kernel tuning
option?

I've had the cable company here and they've replaced the line from the
street and the cable modem itself. Perhaps it's somewhere else on
their network?

How can I determine what those packets were that were dropped and why
they were dropped?

Regular ping tests and tests like speedtest.net work just fine and
show no latency or lost packet issues. DNS tests frequently fail or
timeout.

I don't know where else to go for help or how to troubleshoot this
further. Ideas greatly appreciated.