AMD ganged and unganged memory access

Last year I did a substantial upgrade on my desktop hardware.
Around that time I started noticing an issue with Ubuntu 9.10.
Maybe once a week or every other week the system would lock
up. Whatever was on the monitor would be frozen, keyboard
and mouse unresponsive, computer off the network, only
rebooting would fix it. If I was doing intensive DVD ripping
with dvd::rip it would happen a lot more often, like once a day.
I never observed this in Windows but I rarely have Windows
up for more than a few hours to play games. Upgrading to
Ubuntu 10.04 didn't fix the problem.

Since I've got some time and I'm doing intensive DVD ripping,
I thought I try to troubleshoot this.

(1) The BIOS (GA-MA790FXT-UD5P motherboard) has a 'load
fail-safe defaults' option to load conservative settings, tried that,
system still locks up.

(2) I looked at updating the BIOS. There's one version more
recent than what I've got on this one year old MB. It's a beta
and when I try to install it the flash application says it's got
a checksum error. Scratch that...

(3) Just cruising around in the BIOS menu I found a setting
for memory control mode: ganged or unganged.
Here's some discussion of what this means. Just kind of on
a whim I went ahead and changed this setting to ganged.

So after about 3 days of heavy DVD ripping the computer has
not locked up. Crossing my fingers. I won't really feel sure the
problem is fixed for a couple of weeks since it's so intermittent.