SQL Server 2005 log size & replication issue

Are you taking regular backups of your transaction logs? Backing up a transaction log automatically truncates the inactive transactions.

See: http://support.microsoft.com/kb/873235
You can set up a transaction log backup in Management Studio.

In our databases, the transaction logs are backed up every 15 minutes and stored in a separate directory to allow us to recover the database, should it ever be required,

In this situation it doesn't work.  Putting the database in simple recovery model and truncating log file doesn't work either.  The only way we find it works is to stop the log reader agent and run EXEC sp_repldone...

I have never used sp_repldone
From http://msdn.microsoft.com/en-us/library/ms173775.aspx  ....
"If you execute sp_repldone manually, you can invalidate the order and consistency of delivered transactions. sp_repldone should only be used for troubleshooting replication as directed by an experienced replication support professional."

Did you once have the database set up for replication? Do you need to have it set up for replication? Have you tried taking a full backup of the database and recreating it to see if that resolves the problem?

I know this invalidate the replication.  After we use that we reinitialize all replication.

The database have replication since 3 years but the problem began since 4 or 5 month.  It doesn't occur frequently but cause downtime.

What we know, when this occur, the log file can growth from 1go to 80go in a half of hour!, even if the database is in simple recovery model.  Full backup + log backup and/or and dbcc shrinkfile has no effect (on the log size).

We are not sure but it seem to appear the there a very high number of small transaction on a small period.