Mark, Thanks for the input. Your suggestion of power issues makes as much sense as anything else at this point. I spent last night uninstalling and reinstalling the balky adapter with various driver versions. Turns out I had a much older driver version on the "twin" machine. Aha, I thought. I'll install that version on the server. Didn't make any difference. The machines are running identical BIOS versions. It's an older ASUS motherboard and they quit making BIOS updates a couple of years ago so both machines are on the final version. Nothing I did, software-wise, including installing *just* the driver and not the Proset software, and managing the adapter via Windows, made any difference to the behavior. So, I just finished swapping adapters in the duplicate machines. Again, nothing changed. The balky adapter works properly in the client machine, and the adapter that was working perfectly in the client machine wakes up, but then loses connection and must be disabled/enabled in device manager before it starts working again.
I finally gave up, disabled the Intel nic, and am using the built in adapter on the motherboard - which also works perfectly. I'd love to beat my head against this wall until I manage a solution, but I just have too much going on right now to spend who knows how many hours troubleshooting when a workaround is so readily available, so for now I'm going to drop it. In a few weeks I'll reenable the adapter and see if it wants to play nice again. Maybe some Windows update will mysteriously fix the issue. Or not. If I do manage to figure it out I'll post back. Again, appreciate the help.
Jeff R.