disclaimer: do all or take my personal approach at your own risk
i am not responsible for any damage that you incur if you do the damage.
the following is just what i would do, as a professional, in my own house
first check to see if you first label the wires at the router, AND the ports on the router
EXACTLY what port goes to what device. then unplug them all
check and see, if i were to plug in that PC into the router using either a brand new Ethernet cord
will it then correctly work?
if so the cord was the problem
if not we go to next step in troubleshooting
move just the 1 wire for the problem PC to a different port on the router
does the (DC) disconnect still happen?
if not, then there is something wrong with that single port on the existing router. an internal electrical short of sorts
if it sill happens, then there is something wrong with your PC
at that point we need to see if you can use a different NIC- "network interface card"
i would get a temp/used "NIC" at a local used computer store. a few dollars out of the pocket will cover it
make sure of your open card slots, does the problem PC need a pci interface, or a pci-e? be sure of your machine needs before you make a purchase
another option is USB
USB-Ethernet NIC adapter that an ether net cable plugs into would work fine for diagnostics
the problem is this is going to start getting expensive from here on out
this usb nic can be 60 bucks in some situations
if your still not able to reach a conclusion get back with us, or if my instructions are confusing let us know