Why does my ethernet cord disconnect on its own

I'll take a crack at this.

First, take note of how long it takes before things go wrong (write it down). Perform this action three times minimum.

Also, see if there are any problems with your internet service in your area, and if so, at what times such actions occur, noting if they coincide with your dropped connection.

Run diagnostics on your router. It may be the router has gone bad. It may also be that it needs a firmware update (check with the manufacturer's site).

Sony seems to be a lot better about being able to troubleshoot their devices, so check to see if there are any diagnostics protocols or software that can be run.

Try finding another ethernet cable and plugging it in to the RJ-45 port.