Ethernet connection lost after few minutes since starting

I just upgraded from Kubuntu 16.04 to 18.04 and it is most everything Ok, but -I think that- the network-manager has a problem now.

In the last LTS version I never lost connection through Ethernet (the only Internet connection with our desktop PC), but the Bionic Beaver loses the connection every now and then, perhaps within few minutes or hours, but several times per day.

Only restarting the PC makes the connection to work again.

When the connection is lost, I note that it disappears any info even mention to any network into the network window and the network manager icon becomes red color.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: xorg 1:7.7+19ubuntu7
ProcVersionSignature: Ubuntu 4.15.0-20.21-generic 4.15.17
Uname: Linux 4.15.0-20-generic x86_64
ApportVersion: 2.20.9-0ubuntu7
Architecture: amd64
BootLog: Error: [Errno 13] Permiso denegado: '/var/log/boot.log'
CompositorRunning: None
CurrentDesktop: KDE
Date: Sat May 12 15:37:10 2018
DistUpgraded: Fresh install
DistroCodename: bionic
DistroVariant: ubuntu
ExtraDebuggingInterest: Yes, if not too technical
GraphicsCard:
 Advanced Micro Devices, Inc. [AMD/ATI] Cedar [Radeon HD 5000/6000/7350/8350 Series] [1002:68f9] (prog-if 00 [VGA controller])
   Subsystem: Gigabyte Technology Co., Ltd Cedar [Radeon HD 5000/6000/7350/8350 Series] [1458:2548]
InstallationDate: Installed on 2018-05-11 (1 days ago)
InstallationMedia: Kubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
Lsusb:
 Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
 Bus 002 Device 002: ID 04fc:0005 Sunplus Technology Co., Ltd USB OpticalWheel Mouse
 Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
MachineType: To Be Filled By O.E.M. To Be Filled By O.E.M.
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-20-generic root=UUID=d1d6ad93-b21c-43cd-8410-782414a5f935 ro quiet splash vt.handoff=1
SourcePackage: xorg
Symptom: display
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 07/17/2009
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: P1.50
dmi.board.name: N68-S
dmi.board.vendor: ASRock
dmi.chassis.asset.tag: To Be Filled By O.E.M.
dmi.chassis.type: 3
dmi.chassis.vendor: To Be Filled By O.E.M.
dmi.chassis.version: To Be Filled By O.E.M.
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvrP1.50:bd07/17/2009:svnToBeFilledByO.E.M.:pnToBeFilledByO.E.M.:pvrToBeFilledByO.E.M.:rvnASRock:rnN68-S:rvr:cvnToBeFilledByO.E.M.:ct3:cvrToBeFilledByO.E.M.:
dmi.product.family: To Be Filled By O.E.M.
dmi.product.name: To Be Filled By O.E.M.
dmi.product.version: To Be Filled By O.E.M.
dmi.sys.vendor: To Be Filled By O.E.M.
version.compiz: compiz N/A
version.libdrm2: libdrm2 2.4.91-2
version.libgl1-mesa-dri: libgl1-mesa-dri 18.0.0~rc5-1ubuntu1
version.libgl1-mesa-glx: libgl1-mesa-glx 18.0.0~rc5-1ubuntu1
version.xserver-xorg-core: xserver-xorg-core 2:1.19.6-1ubuntu4
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev N/A
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:18.0.1-1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.917+git20171229-1
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.15-2