Not sure why try checking the SCSI cards speed settings I have seen some act differently with to high of a speed setting. Also update all firmware - but I believe you have this setup. I know you have set this with the cache enable, please make sure of this with the manufacture.

Look at the test.log in the hdparm -t /dev/sdb section to check the Timing buffered disk reads.

Check memory info in section of "cat /proc/meminfo" to see if
Check NIC's if there are any rx or tx errors from "ifconfig -a"