I have the same problem. But the modem only disconnect one time by day.
I have this error message in the log :
daemon.notice pppd[1562]: Modem hangup
I'm using a speedtouch ST510 modem with firmware 4.2.10.0.
The modem is in bridged mode, openwrt 0.9 is in pppoe mode.
The modem report a clean adsl link, without excessive errors nor DSL sync or noise problems. The link is working nicely at full speed everytime.
Normaly the pppoe link should not disconnect as i have a fixed IP.
The only way to get things working again is to reboot Openwrt. A ppp if down - up does not solve the problem. The Internet connection is not totaly lost after this error message, but it seems that openwrt after the modem hang up does not reset correctly the iptables, causing strange connectivity problems.
Strangely i'm using another box with the same setup with the same modem on a dynamic pppoe link without problems. It seems that there are hang up problems only with a fix IP.
Olivier.