"Network Cable Unplugged"/Phoneline Network Problems

I have had a 10Mbs phoneline network for quite a while. One computer runs Win XP and acts as the server for the cable internet connection. The phoneline card in the computer is directly plugged into a wall phone plug, and the other computer networked to it (runs Win 98 SE) is plugged into the the same wall outlet with another phone line connected to its card. This has worked fine for over a year! Suddenly, I started getting the dreaded "Network Cable Unplugged" message and the client computer lost connectivity to the server. Since this is a phoneline network, there really aren't any true network cables other than the phonelines, which are working perfectly! I even plugged a Linksys phoneline USB adapter into the server and even though it lights up, I still get the same message. I tried updating drivers, etc., to no avail (the network had been down for about a week). This a.m., it was suddenly working perfectly for a few hours, then suddenly displayed the "Network Cable Unplugged" message again and the network is down again! NOTHING had been touched or changed either before or after it worked briefly. I am convinced that either:

1) this is some sort of software/conflict issue, or

2) this is some sort of intermittent hardware functioning issue.

Any help would be appreciated!