I just started having a similar problem after downloading the latest patch an hour ago, and then playing for about 25-30 minutes. My screen started freaking out, and my PC ultimately locked up (though I could hear the Guild Wars 2 music in the background, even though the screen had gone black by that point). I forced my computer to shutdown.
My computer and my fiance’s computer just did the same thing as what your describing here. Mine locked up first followed by my fiances a few seconds later.
My pc specs:
i7-3770
GTX 560 2GB superclocked
16GB ram
2tb hard drive
His pc specs:
Phenom II 3.4GHz (2 core)
GTX 550 ti 1 GB
5GB ram