LifeHacker has an interesting article about a "secret" diagnostics page that's made available by your cable modem.
For my cable modem, a Motorola SurfBoard SB6182, I found a number of pages at the recommended URL. There is some interesting stuff, including a log showing DHCP renewals, startup information, etc.
I think it's important to look at the comments on this post. There are a couple of people that have given some descriptions of the numbers shown and how to interpret them.
You can find the LifeHacker article here.