Hi *SM'ers !

I'm facing kind of strange behaviour of our Informix clients : everyday 5 AM 
informix servers are doing  a level 0 (complete) backup of their Db, that goes 
directly to tape (LTO drives, in a 3584 library, connected on an IBM SAN data 
gateway 3108 G07 model) through a fiber channel network, and the problem is 
that the transfert rate varies randomly from an approx 7000 kbps to a 35 !!! 
kbps.
Problem does'nt seem to come from the network, because the same machine at the 
same time is able to send its logical logs at a normal transfer rate, but on 
ssa disks this time.
Did anybody face the same kind of problem, or does anybody have an idea to test 
or improve performance ?
Thanks a lot for any precious advice !
Arnaud