In case anyone has a similar issue, I think I've fixed it. Here's what I did (in addition to above):
Turned off IM programs (msn and trillian)
Turned off Progress Quest
In advanced video options in game, turned off 'new water'
I'm testing this in previously unplayable areas and it seems to be running just fine. Not sure exactly which one of those fixed it, but it's something to try.
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