This began by the computer all of a sudden re-booting on it's own when I was in the middle of a program or game. The screen would just go black, I would see the "no signal" message from the monitor followed by a re-boot, Scan Disk, etc. This occurred about a dozen times over 5-6 days.

Today I can't even get the computer to load the OS. When I turn it on, Win98 begins to load until it gets to the point of displaying "HIMEM is testing extended memory" - then, after a few seconds it displays an error message about unreliable XMS memory at address 0382B278 (although the number and exact test changes from time to time).

I have a 1G AMD with 512 MB RAM running Win98SE. I have tried - without success - the following:
1. Using different sticks of RAM, one at a time.
2. Replacing the AGP graphics card.
3. Booting without any PCI cards.
4. Pulled out secondary IDE cable.
5. Switched IDE cables.
6. Removed/replaced CMOS battery.
7. Checked every connection on motherboard, including removing/replacing the CPU.
8. Using 4 different (backup) hard drives - all of which I know were working fine, but now all of them produce the same result.

So, hardware problem? Motherboard?

Any suggestions or assistance will be greatly appreciated.