FF14 is particularly picky about dropped packets. It can recover from dropped packets, but not well; which means if you are having any issues at all with speed (i.e., play during peak times) it can cause problems. I had similar problems while playing. The solution for me was to bypass my wireless and use wired. Other people have reported that upgrading to a more modern, high quality wireless router, solved the issue for them.

Now, when ATT freaks out (as it frequently does) FF14 is the only that that doesn't wig out. I've had DNS go down before, preventing me from doing anything in firefox since I don't know the IP addies of the web pages I visit, and Final Fantasy 14 clunked right along without an issue. This is actually a regular issue for me (ATT seems to be having issues after 8pm PST.)'

In any event, try wired, see if that helps if the other suggestions don't.