Thanks for the info!
First: I'd try running 8.2. Betas are usually pretty stable, but I never run them except for testing. In your case, since you are having problems I'd try 8.2 before trying to spend time "fixing" the issue.
Second: Have you run any diagnostics on your hard drives? If one of them starts having problems it can result in bizarre performance and reliability isssues. I have WD20EARS running with no problems so I know the drives should work fine.
You can try using iozone to see how your io is going on your pool. If one drive seems to be lagging behind the rest that may be a problem.
I know from personal experience with backing up across the LAN that some programs do not do it very well. Acronis is riddled with problems trying to backup to/recovering from the LAN. It's been an issue for 3 years in a row and alot of customers have left Acronis for alternate solutions because it doesn't work(including myself). Their forum is a good testament to their problems. Acronis has gotten some users to upgrade to 2011 and then to 2012 claiming they fixed the issue, but it hasn't been fixed. I won't be going back to Acronis until I can get a trial version to work flawlessly for LAN backups, assuming I ever have a problem with O&O backups(which work great over the LAN!)
What happens if you backup to a local hard drive and then copy the file to the server? If it works fine then the problem sounds to be related to Symantec's implementation of LAN backups. How big will the backup file be when the backup is complete? You aren't trying to backup across wifi I hope ;)