It's a bit early to be declaring it fixed, I think, but for the moment anyway it appears to be relatively stable again... ran a system restore to some point yesterday around noon, and it hasn't fallen off yet... just keeping my fingers crossed.
A bit ago it appeared that it would manage to reconnect itself to the internet any time I let it alone for a while and came back to wake it from the screensaver, only to disconnect again a minute or so later... which would appear to substantiate the theory that it's a software-related problem.