This description is typical of a noisy or marginal quality cable. I have seen this when something else is attached to the same cable, or
when a connector on a cable end is going bad.
Seems that solution is hardware one, and thus kind of off topic. Oh well.
After reading yours, Marc's and Michael's replies, I went to my basement and rechecked the cabling. What I found was that my father in law disconnected the TV in the basement (we are doing some renovations upstairs, and he needed to move it to get access from bellow). So one of the cable branches was not terminated anymore. He did it couple of weeks ago (we don't use that TV much, so it went unnoticed), and I didn't had problems with it until couple of days ago. I replaced three-way splitter with two-way splitter (so that all branches on the splitter are active and terminated), and now everything seems to work stable again.
Could it be that dead branch on the splitter affected other branches, and that signal quality was just barely good enough for modem (and it needed just a small change somewhere else to go over the edge)?
I'm not sure if this was the reason, since it seems as the things were starting to normalize again even before I eliminated the "dead" branch. If nothing else, at least the modem will get higer quality signal now that it is on two-way splitter.
Or it just might be oxydation on the connector. When I unplugged and plugged it again, oxydation gets scratched off. If it starts failing again in couple of weeks or months, and unplugging and plugging helps, it might be that I need better connectors... Gotta love troubleshooting without proper equipment to measure signal quality ;-)
-- Aleksandar Milivojevic <amilivojevic@xxxxxx> Pollard Banknote Limited Systems Administrator 1499 Buffalo Place Tel: (204) 474-2323 ext 276 Winnipeg, MB R3T 1L7
