Does your DSL modem support SNMP?
I use a D-Link DSL-504 modem for one of my connections, and can accurately monitor the connection quality. While it doesn't have a timer for downtime, the MRTG graphs show when the attenuation/SNR changes, or when traffic in/out hits 0bps.
If you use an Alcatel Speedtouch Home or Pro, you can telnet to the modem, log in, and issue the 'adsl info' command for a dump of the current status. Roll this into an expect script to feed MRTG and you're set. I may even have such a script stored away somewhere from testing previously.