Some clarification…A cable modem isn’t really “always on”. Just like a regular telephone modem, it goes through a connection sequence which involves some negotiation. First it has to locate the channel the downstream QAM signal is on and lock onto it. Then it has to start talking back upstream louder and louder until it gets an acknowledgement back from the server. On some of the cable modems I have worked with, after a total reset, this can take 5 minutes. The reason that people consider a cable modem “always on” is that under normal circumstances, this connection is very robust, and it’s never shut down on purpose. So under good conditions, it’ll stay connected all the time.
If you’re getting a really bad signal, it may very well be losing the connection and hunting for a new channel to lock onto, which can take a while. Depending on the cable modem type, this could be indicated by a blinking front panel light. If your cable modem is keeping a solid connection, the problem is probably with your PC’s networking setup, and the rest of this advice probably won’t do you much good.
One thing to check first is all of your connections and cabling (the coax cable-tv cabling, that is – not the connection between the PC and the cable modem). Strange as it may seem, using cheap-o cable splitters from radio shack is very frequently the problem. Not to worry, the “premium” ones that radio shack sells are just fine (generally, it’s a sign of quality if they have signal loss specs printed on them – a good 2-way splitter will say “-3.5”, indicating that 3.5dBmV of signal is lost going through the splitter. If it doesn’t have a number at all, or the loss is greater than 3.5 or 4, it’s probably crap). They’re a couple bucks more, but worth it (you’re likely to see an improvement in picture quality on TV too). Also, make sure you’re using RG6 cabling, not RG59. RG6 is fatter and better shielded. Lots of people buy a roll of RG59 and try to run RF video over it, and introduce lots of noise. If the cable is about 1/8 inch thick, it’s probably RG59, and is made for baseband video, not RF. Usually, the cable will have some lettering printed along the length with a bunch of numbers, and it’ll say RG6 or RG59 somewhere in there. If you crimp your own connectors on the cable, don’t skimp. Get the good ones, and get the good crimper tool (within reason – you don’t need to go with the 125 compression tool with connectors that cost .50 apiece unless you’re setting up your own cable plant :))
A sanity check you can do is take an extra TV if you have one, and unplug the cable from the cable modem, plugging it directly into the TV. Make sure you get a good picture from that tap. If the picture’s good, it doesn’t prove everything’s ok, but if the picture’s bad, you know something is wrong with the signal somewhere. After that, follow the cable back to where it comes in the house, if you can, checking all the connectors along the way. Frequently, someone’s done some lousy job splitting the cable off to an extra bedroom with a crappy splitter, and is introducing noise.
If your connections all check out you can try to get the cable company out there to measure your signal levels to make sure they’re in the acceptable ranges. If you fib a little and say you’re getting a fuzzy picture intermittently or when your connection drops, they might bring a scope out and make sure everything’s hunky dory. It’s possible you have an amplifier in your neighborhood acting up in a way that’s mild enough to affect cable modems but not TV, and you’re the only CM customer on your block.
Sorry for the long post. I spent about a year punching and kicking my very own cable modem plant, and damn it, somebody’s gonna benefit from it, even if I have to bore you all to death in the process.