Label says Hardware version is 3.8 but none of the hardware version numbers available above match 3.8.

Anyway... I have internet service that measures 350 Mbps directly from the cable modem. It also measures 350 Mbps from 3 different routers that are in my home network using just an Ethernet cable to connect the computer. This particular laptop has no Ethernet port built-in, so I am using a USB 3.0 1Gbps Ethernet Adapter. So "Ethernet speed" measurements made with Speedtest.net are also using the USB-Ethernet adapter. I can measure my desktop computer at 350 Mbps, then disconnect that Ethernet cable and use it with the laptop and also get 350 Mbps.

I have 4 2000 series adapters. 3 are in use at the moment. One is connected to the same router my desktop is connected to. One adapter is connected to a work laptop and it measures 340 Mbps.

The third Ethernet Power Line adapter is in our Family Room for this personal laptop that measures 350 Mbps connected to the same cable as my desktop computer in another room. The Ethernet connection in the family room has been measuring 330-340 Mbps. But for the last 2 days, I can get no more than 40 Mbps. I have unplugged all the power line Ethernet adapters with no change. And the speed/performance of this laptop when connected directly to Ethernet (not over powerline) is PERFECT. Over powerlines, this laptop WAS perfect, but has suddenly changed for no reason I can detect.

Any ideas to try?

Already on the list: pair the 4th power line adapter and see if the speed is any better in the same room with the same laptop.