We should ask badspot about this. Have you tried putting your 590 back in and seeing if theirs a difference?
Turn down your draw distance. The Block Party probably has one of the highest brickcounts, so that would probably help.
When in the server, press Ctrl + N to display the Net Graph. Tell us your average latency.
Does this only happen on the block party?
I think your GPU just might be defective, make sure its installed all of the way in the slot correctly if it is I would see if you can return it.