Log Shipping Fails Once a Week

Hi
I have log shipping setup on 2 servers - A and B. Both are SQL Server
2000 and the size of the database is 95Gb. The average transaction log
is around 100Mb
A ships to B and B is also the monitoring server.
The schedule is set to ship every 15 minutes.
Everything works fine, however every Sunday at 23:00 without fail the
log ship fails due to the LSN counter being out of sync. The error
reports an ealier backup is available but any attempt to restore fails
and the log ship schedule must be deleted and re-created.
There are no other backups/restores or any other scheduled jobs that
occur at this time.
Does anyone have any suggestions?
Thanks.
What is the exact error message you receive? Please can you post it up for
us...
Cheers,
Paul Ibison