First thing to try is to get a brand new Ethernet cable for 8920.  Physical inspection of cable does not rule out hidden cable issue.

I'll certainly try that - and report back here 

@Ron D in France   - Does "restarting" the PC mean doing just a Restart in Windows or a full power-off shutdown?

Any signs of overheating?

Have you looked in Windows Event Viewer to see if any errors are logged around the time of lost connectivity?

What happens if you use WiFi on this PC instead of Ethernet?

When was last time you replaced the motherboard battery, CR2032 3-volt coin cell?

Hi RoHe,

> Does "restarting" the PC mean doing just a Restart in Windows or a full power-off shutdown?

I tried BOTH

> Any signs of overheating?

NO 

> Have you looked in Windows Event Viewer ...

NO - I've never used it (didn't even know it existed!!) - but I'll try to do this and report back

> What happens if you use WiFi on this PC instead of Ethernet?

Same - no internet

> When was last time you replaced the motherboard battery, CR2032 3-volt coin cell?

I've NEVER changed it - didn't even know of its existence !! - what's the normal life?? I installed this system 19 May 2017. Is it easy to do for a 'simple' user like me??

Did you configure the XPS 8920 to access your WiFi and then disable the onboard Ethernet and enable WiFi in Device Manager when you want to test the WiFi connection?

Motherboard batteries may last +/- ~3 - 4 years but YRMV. If you disconnect PC from the mains or turn it off at a power strip at night, that can shorten battery life. Since your battery is 4+ years old, this might be a good time to replace it, if nothing else than to rule it out as part of the problem. I see them on Amazon EU for ~€2.50 - €4.50. Instructions to remove/install the battery are in the XPS 8920 Service Manual.

And see what the Windows Event Viewer reports, if anything, around the time you lose Ethernet connection.  It doesn't sound like Windows is just turning off the Ethernet to save power because rebooting the PC should automatically turn it back on.

So if replacing the motherboard battery doesn't help, and there's nothing useful in Event Manager, you can try running these commands in case there's a Windows software issue:

1. At desktop, open a CMD prompt window, Run as administrator
   
   
2. Type in: chkdsk c: /r and press Enter
   Accept offer to run chkdsk at next boot and reboot PC
   chkdsk will run before Win 10 loads so be patient and note any error messages. And you can also look at the chkdsk log
   
   
3. When you're back at the desktop, open a CMD prompt window as in #1
   
   
4. At the prompt, type in: DISM.exe /Online /Cleanup-image /Restorehealth and press Enter
   Be sure to include a space in front of each / and note any error messages when that's done.
   
   
5. Assuming no errors in #2 or #4, at the CMD prompt, type in: sfc /scannow and press Enter.
   Be sure to include a space in front of the / and note any errors when that's done.
   
   
6. Assuming no "unfixed" errors in #2, #4 or #5, reboot and test Ethernet again.

Hi redxps630,

Thanks for this. As you'll see from my reply to RoHe, I appear not to have correctly tested the WiFi connection, so next time I lose the Ethernet connection I'll follow RoHe's guidance and report back.

Re the battery, I close down Windows and switch off the power at the power strip every night - I guess that's just habit which started in hot summer to avoid overheating in my little office with no aircon. I hadn't realised it would shorten cmos battery life. 

Hi RoHe,

First a big THANK-YOU for all he help you are giving me - I really appreciate it.

>  Did you configure the XPS 8920 to access your WiFi and then disable the onboard Ethernet and enable WiFi in Device Manager when you want to test the WiFi connection?

NO - I didn't - I merely selected the WiFi n/w connection from the n/w list. I realise from what you have said that this was not enough! I'll wait until it occurs again then do as you propose.

I should also mention that I replaced the Ethernet cable yesterday morning as suggested by redxps630, and the problem has not occurred again since (but that doesn't necessarily mean it's fixed as it was sometimes three days between occurrences, so we shall see). I'll also wait for the next occurence to check the Windows Event Viewer with a precise time of the event to help search the report  - there's a lot of stuff in there so not easy withut a fairly precise problem start time (which I didn't note before but will next time).

As for the motherboard battery, I'll certainly replace it as you suggest - thanks for the tip. I wonder how many users who keep their PCs for more than 3-4 years like me are blissfully unaware that this is something that should be considered every few years!

Thanks for the set of test recommendations for checking for potential Windows issues. I guess it would be better to wait until the next occurrence to run those too.

Thanks again. Will report back here when all the above done.

If you lose wireless internet intermittently too then it sounds more like a software issue with your OS.  it would be hard for both gigahbyte ethernet driver and separate wireless driver to go haywire at same time.  try use a spare hdd to do a fresh clean install of Windows 10 (remove current ssd/hdd from system for now) and test system network stability on the test hdd with clean OS.  If you do not have a spare but are willing to lose old data, wipe the current hdd and do a clean install.

if your 8920 is plugged into power all the time, then the cmos battery issue is unlikely to affect network stability.  the cmos battery is intended to keep nonvolatile memory in cmos chip when pc is complete off power (power cord unplugged).  when ac power is connected, the system is automatically on standby power even when pc is off.  that is why in some dell pc a yellow light is always lit on the motherboard when power cord is plugged in.

@Ron D in France  - You're welcome. 

It is puzzling that doing either a Restart in Windows or a full reboot doesn't bring Ethernet connectivity back, which is why I asked about the motherboard battery. 

BTW: What browser are you using? Sometimes Firefox locks up on my XPS 8930 and I lose connectivity with both Ethernet and WiFi, but the PC still runs normally, otherwise.

Closing/restarting FF or switching to a different browser doesn't fix it. And doing a Restart in Win 10 doesn't fix it either, but doing a full power-off reboot always does.  So similar symptoms to yours but a different response after rebooting...