Check the SQL error log. Are you seeing recovery model changes? Do you have any other scheduled jobs that do anything to the log (including truncate it)? Do you have any 3rd party backup tools running against this database?
What version, since it's clear now that it's not a SQL 2000 database?
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability