FreeBSD 6.1 kernel panic

[freebsd at top-consulting.net]

I have the following setup:

3ware 9550SX-4LP raid card

4x 500GB SATA HD's in Raid 10.

It's running on: FreeBSD store01 6.1-RELEASE-p12 FreeBSD  
6.1-RELEASE-p12 #0: Mon Jan 22 11:20:03 UTC 2007

I use this machine mainly for backups and NFS mount points. The  
problem is that when I perform backups at night, within 1 hour or so  
the machine reboots because of kernel panic. The exact error message is:

Mar 31 00:29:05 store01 syslogd: kernel boot file is /boot/kernel/kernel
Mar 31 00:29:05 store01 kernel: start = 0, len = 11431, fs = /usr
Mar 31 00:29:05 store01 kernel: panic: ffs_alloccg: map corrupted
Mar 31 00:29:05 store01 kernel: Uptime: 2d22h57m30s
Mar 31 00:29:05 store01 kernel: Cannot dump. No dump device defined.
Mar 31 00:29:05 store01 kernel: Automatic reboot in 15 seconds - press  
a key on the console to abort


This has been happening only since I started doing massive backups on  
this box ( lots of directories/files ). After reboot, if I run a fsck  
-y on the partitions everything turns out fine but with "fixed errors"  
like: Mar 28 03:53:13 store01 fsck: /dev/da0s1f: INCORRECT BLOCK COUNT  
I=28342913 (40 should be 0) (CORRECTED)

The only other place where I've found a problem very similar to mine  
is:  
http://www.nabble.com/kern-35580:-creating-directories-on-large-UFS2--%3E-boom-t3203420.html  but that is on  
NetBSD.

Does anyone know if what I'm describing is a current bug in FreeBSD  
6.1 or a problem with the 3ware card or something else.

Thanks!