when I think of "machine locking up while I am away" I am thinking of power saving managment issues.
Try disabling all power managment stuff in your BIOS. Try passing (I am thinking that these are the correct parameters) acpi=off and apm=off to your kernel at bootup time. Also check to make sure that you have only one of apcid or apmd running at any time.
See if that helps it go away. maybe the BIOS is trying to shut off your vid card or cause your harddrive to spin down and the hardware doesn't like it.
For the serial cable stuff you need to make sure that you use the right type of cable, too. They are called 'null modem cables' or 'laplink' cables (after a namebrand). They are used, i beleive, in external modems, so if you have one of those you could probably use it's cable.
If you use the wrong type of cable you can blow out some circuits on your motherboard. (i had a old boss blow out a 5000 dollar RIP thingy for a very expensive printer once doing that, warned her but she didn't listen to me. Live and learn)
Very usefull for servers. Say you have a dozen servers on a rack, you could setup one monitoring station with a multi-port serial card hooked up to all of them. Lilo, and maybe Grub, supports redirecting your main console output directly to the serial port. Then you have a bunch of minicom sessions going on and you can see the output for all those machines. Very usefull. Especially for security. You could have all root access denied to the servers except thru those serial ports for instance.
I like to use a old laptop with battery issues as a monitor for servers.. you have 2 serial ports usually. Lot nicer then having a bulky old monitor and keyboard with a few broken buttons that nobody wants attached to each and every server.