How old is this PC-- any warranty ?
If so contact the seller/
mobo maker-- see if it's covered.
If you don't want to bother for a network card--see below
Probably the simplest fix for this-- is to install a PCI NIC-- if you have an open slot-- they are not much money.
You have tried almost every hardware troubleshooting trick I thought of already... I would begin to suspect the adapter itself. Simplest just to bypass it. I would suggest at least a gigabyte adapter these days.
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