Possible silly question/input incoming:

Did you check 'No Frame Rate Limit' under System Configuration? If not, even set at 60fps, you might see issues. This setting is notorious in the past.
Otherwise, the suggestions other people have are great. MMO's tend to be more CPU-driven than regular games, but either way you should be covered there. It is also possible that you have a poor connection. Try this, if you haven't already:

http://na.finalfantasyxiv.com/lodest...4/forum/43814/

I used to have a very similar problem and it was related to a bad node owned by Verizon.