Well, we've finally got our DSL back up. Apparently it only works when the ceiling fan light in the dining room/office is on. It's not the entire fan that does it, either, just the light fixture. Turning the light off with the pull chain while the fan was on had the same effect as the wall switch that turns off the whole thing. Tested by loading a page with a large graphic, progress on the image stopped dead the instant the light turned off, then worked again once it was turned back on.