I have the Comcast 50Mbps tier (before speed upgrades, I don't believe I've seen mine yet) and have had pretty good speeds before the last couple weeks. I never really paid attention to the modem (owned, D-Link DCM-301) but I decided to take a look. While the stuff displayed seems to be within specs of the FAQ, I noticed that 4 downstream channels are "Unknown" and the uncorrectables seem to be high for a modem that I only rebooted 10 hours ago (and the counters were near zero). When it came back up, the speed seemed to be normal for an hour, but quickly went back to being unreliable. I've attached screenshots of the modem status pages to this post.
I should follow up on this. I did quite a bit of testing with my connection and narrowed the culprit down to an old splitter that has a -5dB loss on each leg. Changing things around a little has made it acceptable and errors occur much less frequently. After doing some additional research, it looks like I might be best served by getting a TAP and using that instead (or just getting a new splitter with -3.5dB loss). I also need to pick up some shorter cables as I'm using some from a previous move and are much longer than necessary.
In case you're wondering, after the splitter, it goes to the modem and a HDHomeRun 3 cable card.
Does anyone here know of a good source for reliable TAPs? I haven't found a whole lot yet.
It would seem like you have an ingress problem, all of the channels that have low to no SNR, seem to be on frequencies that correspond with Denver OTA stations. Check for loose fittings.