After seeming to run fine, the problem returned. Every so often, I would logon to the server to get an 'Unexplained System Error' which forced a reboot of the server. Over the next few weeks, I brought the temperature down, which seemed to help. I then told BESR to break the backup into 50GB chunks, which seemed to help a little more. The problem would reoccur intermittently, however. Symantec wouldn't help me because I hadn't purchased their support, even though this has been an issue since it was new.
I then got in touch with Adaptec, who had yet another firmware update available. Between the latest firmware for their controller and the smaller backup files, it has been working for the past 2 weeks without error, even with the temperature raised back up. They also have a newer Windows driver for their controller, but I haven't installed it as of yet.
I'm keeping my eye on it.