Something like this happened to me few time ago and I use SLES, suspected kernel probs, processor, memory, did a bunch of testing and i couldn't get the cause. Memory tests were running through the night and still no problem. But backuppc does stress up the components, encryption, compression, heavy IO, i bought some new ram (the most inexpensive item) and started changing exchanging hardware, fortunately it was the ram and as it still was in warranty they replaced it for free. Since then no probs.
The funny thing is that faulty ram didn't hang up or gave errors in memory tests, while proving i would have some errors compiling the kernel, but never had problems outside of backuppc.
My advice is that you should check your hardware, and even other conditions as temperature.
If you want to test with other besides backuppc try to compile your kernel (not for usage but as a stress test).

On Thursday 30 April 2009 17:51:05 Christopher Derr wrote:
> I'm currently running the latest backuppc version that Ubuntu officially
> supports (it's behind Debian as far as I can tell and I haven't tried to
> use the Debian version): 3.0.0. Apt-get shows it's the latest available
> through Stable. Anyway, the system (Tyan 2912G2NR, 8 GB memory, 4 TB in
> RAID 5) crashes often. Becomes untouchable, I go to the console, hit
> enter to bring up a logon prompt, then the machine is officially
> frozen. I figure it's a kernel panic, but I'm not seeing anything
> telling in any log I can find. This happens almost exclusively when
> backing up a one of our Windows fileservers using rsync with 700 GB+
> data over our 1 Gb link.
> My thoughts are it could be Ubuntu or it could be the hardware. They
> system doesn't seem to have any issues except on this one machine's
> backups, and even then it's not every time (just most of the time). I'm
> considering moving to Debian and the latest version of Backuppc (3.1...I
> realize 3.2 is still in beta). I think I just need to backup my
> /etc/backuppc config files but if I keep the /var/lib/backuppc mount, I
> should be able to reinstall the system without affecting the backups.
> Not sure if the 3.1 upgrade is going to talk with the 3.0 backup files
> though.
> Thoughts about Ubuntu or my upgrade in general?
> Thanks,
> Chris
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Pedro M. S. Oliveira
IT Consultant
Cellular: +351 96 5867227