Yeah so from almost everyone I've seen with this hiccup issue have been using a wireless network adapter, and if they go hard wired it seems to resolve. I don't know why but disabling QoS packet scheduler seemed to work for some people including myself, but others had to disable HID devices, if neither of those are working, I'm not sure what to tell you other than to stay hardwired.
The issue is def. only ffxi/polviewer. You can really see it in the ticker that goes across the bottom on the screen where you press "play ffxi", and it's extremely annoying in game.
I can't pinpoint the issue to the finest detail but I'm fairly certain it's a vista/wireless issue. I wish I could be more help!!
(I also used Linksys wireless when I had this issue just fyi)