When my ISP and I started trying to fix the DSL connection the first thing they did was send me a new replacement modem. I swapped out the old one for the new replacement, but it didn't fix the problem so we figured "must not be the modem" and we went off on a snipe hunt involving data caps, fast/interleaved rates, two AT&T visits, and an rather expensive internal rewiring of the house.
The AT&T guy visited this week to test the lines and reported that, if anything, my phone lines are exceptionally good. He also added that "I've seen a lot of problems with this silver Motorola modem". I told him about the impossible "-17.-20 SNR" errors and he said yeah, you might as well just replace the modem with something like 2Wire, which is pretty reliable. I went on Craigslist and found a guy selling a 2Wire 2701HG-B. Plugged it in, spent about 15 minutes figuring out how to change its settings to work with our network, and suddenly we had total stability. I had two bad modems.
Sonic.net says that they've been aware of the problem for a while but are continuing to use this model because Motorola fixed the problem. They say that serial numbers beginning with 157 don't have the bad capacitors. Except that the replacement that they sent me has a 157 serial number, and it was also broken, but I don't care, because I've got my own 2wire which is very stable and reliable now.
In conclusion, if you see this modem, kill it with fire.