Up until last night for some reason FFXI has been lagging on my PC. Turns out it lags for about a second every four seconds. Any one have any idea on to why it is doing that?

-I've checked FFXI files and everything was good.
-Checked to see if some how I got a virus and I'm clear of that.
-Reseted my modem and wireless router and still hasn't fixed. (netgear wireless-G)
-Could it be that my PC is finally giving out? its about five years old.
-Doesn't happen when i use other programs.

I'm about to travel to get my laptop and reinstall ffxi to see if the issue is the net or the PC