6850 Random Lockups / Red Hat Linux AS Release 4

We have a 6850 running Red Hat Linux AS Release 4 that randomly locks up under heavy I/O load.  All current patches are installed.  The system is used as our backup server and there is very little CPU load.  We using Netbackup Enterprise Server ver. 6.0MP4.

 

System has:

8 LTO 2 drives connected to 3 ATTO ExpressPCI UL5D LVD SCSI controllers.

2 DLT 700 w/ Stk L700 robot attached to 1 Sysbios HVD SCSI controller

 

Has anyone ran into random system lockups?  There are no log files written, not on messages file or in the DRAC logs as well.  Any info would be helpful. 

 

Jim