FFXI - Game randomly shuts-downFollow

Processor: AMD Athlon(tm) XP 2500+, MMX, 3DNow, ~1.8GHz
HD: 80 Gig Western Digital, partitioned into 4 drives, E: containing 20 Gigs, 11 Gigs free space.
Memory: 512MB RAM
Card name: RADEON 9600 SERIES
Operating System: Windows XP Professional (5.1, Build 2600) Service Pack 1 (2600.xpsp2.030422-1633)
System Model: AWRDACPI
Frequency: Happens at a minimum of once per gaming session. During that session, it shuts-down at random.

I'll be playing the game, sometimes for a long/short time, then suddenly it'll just shut down. Both PlayOnline (required to run simultaneously with FFXI) and FFXI itself will just close. Once, it shut down, causing me to re-launch the game, but as soon as I entered the world of FFXI, it shut down again with me even touching anything (right after the world/area finished loading). So: shutdown, re-launch, shutdown.

Other times, it would shutdown, then I would re-launch the game and play for hours and it wouldn't happen again.

I've done a clean install (reformat/reinstall) of Windows XP, downloaded SP1, gotten the latest drivers for all hardware.

I'm not sure what else needs to be said. I know, for a fact that there aren't any viruses because I've run virus scans using the latest Norton Antivirus virus definitions. I've run SpyBot and Ad-Aware and neither found any spyware.

I've tried using a GeForce Ti4600 to see if it might be a problem with the graphics card, but this time, instead of shutting down, the system seemed to crash. I would be running through town, when all of a sudden a blank, black screen appeared, then it flashed to a blue screen, then the system restarted.

Sorry 'bout such a long (and possibly confusing) post. Just trying to show which possible-solutions I've attempted. I've even emailed Square-Enix about this and followed their directions (opening ports in the Router, dl'ing latest drivers, spyware/virus scans, etc.) to no avail.

If any kind of further advice or suggestions could be given, it'd be greatly appreciated. Thank you!