I was just about to link/suggest a powerline adapter.
I'm presuming you have a cheap switch or router acting as a switch downstairs. Out of curiosity... do you make your source IP address (computer or NAS) static (tied to the mac address of your source)?
If this is addressed to me, I have no idea what you're talking about
If so, I just plugged one in upstairs near the router and connected an ethernet cable and the other plugged into a basement outlet
It was
What I've found in the past... routers tend to assign local IP addresses dynamically. Cycle power, and your computer has a new IP address. This was especially annoying when I had my squeezebox. When I set up the SB3, it would ask for an IP address of the music server on my PC. Invariable after a PC reboot, and music would stop, I'd then realize "Oh... I need to update the setting in the SB3".
After one or two times of that, I logged into the router, determined the mac address associated with the PC, and reserved a specific IP address for it, which was then populated into the SB3. From that point on... everything has been rock solid.
As for the power line adapters... typically, you'll want to ensure they are on the same/similar power leg/service (A or B) (Edit: Doh! You just said that above). I'm currently using a Zyxel model which has been great... I hear the Netgear ones are also very solid.