might not be the same thing, but my firewall decided to limit the bandwidth of all my programs during online access for no apparent reason. I had to reinstall Zone Alarm, then everything was fine for about 3 weeks ... then it happened again.
damn Zone Alarm ... this is the third time its done something stupid. It's my legal version too

I went back to using Norton and have had no problems since (other than the pop up that says you have been updated and windower stops you from getting booted during that ^^ )
Might also try to turn off any routers / modems / gateways you have. Then turn them on one at a time until they completely start up and give the 'green' lights. Start from your PC or PS2 and work backwards to the modem/gateway.
-PS2 or PC -> ON
-wait for green
-Router -> ON
-wait for green
-Modem -> ON
-wait for green
Check for any sibblings that may be downloading something or any bandwidth viruses you may have as well. Don't alway rely on anti-virus software to find them because some viruses fool AV programs into not being able to find them if you are already infected. Check all the processes in your process log ^^
I have never played FFXI on PS2 but I have played other games ... sorry I can't be of more help on PS2 ^^
good luck