That's the log I mean when I refer to error log. It's just a hope that the router might occasionally log something useful to reveal the error.
Some routers have error stats (FEC counts) etc. available as well via a CLI.
For instance on my variant of the DG834G I can telnet to the router and issue the command
cat /proc/avalanche/avsar_modem_stats
to get additional information. Other variants of the Netgear expose the same information using different commands.