Guild Wars 2

Since the release of HoT, I have been experiencing extreme stuttering and low FPS in story mode and PvP. Everything starts fine, but the problem usually begins several minutes into the round or the story. This happens with both 64- and 32-bit clients and seems not related to graphics settings (which are currently set to medium). Normal PvE and WvW appear unaffected. I have tried several older graphics drivers and every possible variant of graphics settings, but nothing seems to help. When this problem occurs, the game is unplayable. Any ideas what might be causing it?

P.S. More experiments revealed the following: after restarting GW2 client, the problem remains. Disappears after restarting the computer, but returns again during gameplay exactly as before. Selecting lowest graphics settings DOES help, but only after the computer has been restarted.

i7 with 12 GB RAM
nVidia 660ti
Windows 10 64-bit