Oracle Database Discussions

Oracle gurus- Fellow in trouble, need your guidance.

I have a serious issue that has no direct error to examine or fix.
I have a replicated offline server that has a scripted RMAN backup to run nightly, but the backup times flux from what should be only 1 hour 30 minutes. Every other day it takes up to 10 hours to complete a backup that normally takes 1 hr 30 minutes. There is no difference in data loads- the server is offline and the data has not changed to any effect. I use database control instead of grid control, and I turn off emctl every night before the rman backup- I thought maybe emctl was hogging resources. There is nothing else running on the server except replication inserting a couple of rows of data on three tables every other minute.

Where would I look to see where the problem exists when no errors are generated?

Does RMAN wait to backup files if they are being accessed?
Here is my backup script (the OS is Linux AS3) I run an AT job nightly.

at -f /path/path/rman_backup 0600 tomorrow
rman target/
run {
backup database plus archivelog;
delete noprompt obsolete;
sql 'alter system archive log current';
}
exit;

The Administrators are running a Legato Jukebox backup? a tape backup that backs up my DB files nightly. They run the Legato backup 3 hours after my RMAN backup- the problem is that my RMAN backup may be running during the Legato backup- so I assume that contention occurs and both backups fight for the DB files. This is not an issue when RMAN plays nice and finishes in the normal 1 hour 30 minutes, but when it exceeds 3 hours- we have serious issues, both RMAN and Legato backups take 10 hours.

Please help, I am really puzzled. This is causing me major headaches, how can I assure that RMAN has a consistent backup time to completion. Is there anyway to script it to use more resources to ensure it finishes quickly?