With all the upgrades taking place these days, to get a better idea of the signal on your end, what you get thru the modem's web interface is in most cases useless. Most modems allow you telnet into them and run built-in diagnostic tools (as long as you know the CLI commands) which give a much more detailed view of what's happening on the line.
DMT DSL Modem Tool, puts a nice GUI on the telnet CLI and makes things much easier.
http://dmt.mhilfe.de/
Note there are different versions for different modems. If you don't find your modem on the list it might still be supported, try to find out what chipset it uses and get a compatible version. Broadcom's BCM 6338/6348/6358 chipset line are very commonly used (DMT v.8.07), that's what I have in my D-Link 2540U.
There is a hidden (enable in settings) experimental section that you can use to force the modem to use lower target SNR margins when syncing, which allows higher sync rates on lines with low SNR, but you trade stability. These settings don't stick and are lost when the modem reboots.
And you can run a SNR monitor that will show you fluctuations through out an extended period on every frequency / bin used on the line.
my results aren't too interesting since my signal quality is way more than enough for 2mbps only. But others might get a better idea of what sort or noise is creeping in on their line, and at what time of day.
Anyway geek out ... lots of data points to enjoy :P