ever since updating to 18.04 ethernet keeps resetting several times per second

[Michael Bloom]

Ever since I updated this system to 18.04, the ethernet has been resetting several times per second.
/var/log/syslog looks like this fairly rapidly. It makes updating awkward. I have to keep restarting the update because it keeps complaining intermittently about being unable to contact the repository, but the system is currently up to date as of a few minutes ago.

syslog looks like this:
Apr 15 16:00:30 notebook kernel: [42489.401932] atl1 0000:02:00.0: enp2s0 link is up 100 Mbps full duplex
Apr 15 16:00:31 notebook kernel: [42490.810245] atl1 0000:02:00.0: enp2s0 link is down
Apr 15 16:00:31 notebook kernel: [42490.819643] atl1 0000:02:00.0: enp2s0 link is up 100 Mbps full duplex
Apr 15 16:00:32 notebook kernel: [42491.281258] atl1 0000:02:00.0: enp2s0 link is down
Apr 15 16:00:32 notebook kernel: [42491.286405] atl1 0000:02:00.0: enp2s0 link is up 100 Mbps full duplex
Apr 15 16:00:33 notebook kernel: [42492.223598] atl1 0000:02:00.0: enp2s0 link is down
Apr 15 16:00:33 notebook kernel: [42492.228567] atl1 0000:02:00.0: enp2s0 link is up 100 Mbps full duplex

1) Here is lsb_release -rd:
Description: Ubuntu 18.04 LTS
Release: 18.04
2) apt-cache policy pkgname: NA, as far as I know.
3) What you expected to happen: Ethernet to stay up
4) What happened instead: It keeps bouncing like atrial flutter!

kern.log and syslog files are filled from top to bottom with lines like those shown above.
Similarly, dmesg -d shows only lines like:
[43956.763221 < 0.005033>] atl1 0000:02:00.0: enp2s0 link is up 100 Mbps full duplex
[43957.230123 < 0.466902>] atl1 0000:02:00.0: enp2s0 link is down
[43957.235245 < 0.005122>] atl1 0000:02:00.0: enp2s0 link is up 100 Mbps full duplex
[43957.701180 < 0.465935>] atl1 0000:02:00.0: enp2s0 link is down
[43957.706210 < 0.005030>] atl1 0000:02:00.0: enp2s0 link is up 100 Mbps full duplex
[43959.115537 < 1.409327>] atl1 0000:02:00.0: enp2s0 link is down
[43959.120522 < 0.004985>] atl1 0000:02:00.0: enp2s0 link is up 100 Mbps full duplex
[43959.586944 < 0.466422>] atl1 0000:02:00.0: enp2s0 link is down
[43959.591890 < 0.004946>] atl1 0000:02:00.0: enp2s0 link is up 100 Mbps full duplex
[43960.058258 < 0.466368>] atl1 0000:02:00.0: enp2s0 link is down
[43960.063293 < 0.005035>] atl1 0000:02:00.0: enp2s0 link is up 100 Mbps full duplex
[43960.529823 < 0.466530>] atl1 0000:02:00.0: enp2s0 link is down
[43960.534784 < 0.004961>] atl1 0000:02:00.0: enp2s0 link is up 100 Mbps full duplex
[43962.415925 < 1.881141>] atl1 0000:02:00.0: enp2s0 link is down
[43962.421103 < 0.005178>] atl1 0000:02:00.0: enp2s0 link is up 100 Mbps full duplex