Onboard Ethernet is automatically active without network card installed.  Unfortunately, there isn't something like a switch that I know of to turn it off & on in BIOS.  You'll have to remove card to test onboard Ethernet.

Also, test your internet speeds with a reliable app (like FCC or cable co. app) with your PC plugged directly into modem.  If you get good download and upload speeds, then you know it's the router.  WiFi speeds can also be tested using router and compare with modem speeds.

Re: Is this possible and how do I enable the onboard network adapter?

Yes it is possible.  you enter BIOS by press F2 when power up pc.  then enter system config > Integrated NIC.  there you click the small box in front of NIC to enable it.  

I just did it on another Dell pc of similar era.  before then device manager shows no gigabyte driver and pc has no ethernet connection.  after that Intel gigabyte driver appears automatically in device manager and pc now has good ethernet connection to internet.

 

 

 

So I stand corrected on not being to switch onboard Ethernet on & off in BIOS.

no worry.  I was wrong many times on this forum before even just yesterday and I learned from others' wise knowledge.

Thanks.  Even though at times it's hard to be 100% right 100% of the time, we'll always do our best with what we do know.

I was able to enable the onboard adapter in the Bios, and after I found the appropriate drivers and installed them...the onboard network adapter showed up in Device Manager, right next to the Netgear FA311.  I haven't tried switching but I'm assuming that all I have to do is move the cable from the Netgear to the onboard adapter and the onboard adapter will work as it's supposed to.  I'll have to test this before I get the next disconnect.

Hopefully no disconnects.  Otherwise, may be card will still have to be removed?

If card is removed, and disconnects are occurring, an internet problem is a high possibility.  This is unless your PC is the only device the disconnects are happening on.

Had another disconnect, switched the cable to the onboard network adapter and the problem went away.  Unfortunately, a little later there was another disconnect so I switched the cable back to the Netgear FA311 adapter.  The problem remained, so I think that eliminates the network adapters as the probable cause of the disconnects.  The next obvious possibility is the cable from the router(upstairs) to this downstairs computer (which is the only one having the disconnects). 

Can anyone recommend a cable tester that's not too expensive?

I'm going to let you take your pick on Amazon. 

Do you have a cable outlet in the same room as the PC that disconnects?  That way, you could try it plugged directly into the modem instead of the router.  Or, if your router and modem are next to each other, switch the cable to the modem.

The disconnects aren't occurring on my upstairs computer (where the router and modem are located)...the first thing I do when there's a disconnect is to go upstairs and check on my upstairs computer.  I guess it's possible that the disconnects are only coming from one of the router ports, so I'll try bypassing it next time it happens.