This has been happening to me pretty often since around the Halloween release, too. It’s memory-leak like behavior, where I drop into single digit frames for a couple of seconds every so often, but never permanently or for extended periods of time.
Usually when it first happens my screen will go black for a few seconds, showing a menu bar at the top as if it were in Windowed mode, and then the image returns and the periodic frame dips start happening.
However, task manager doesn’t seem to show an abnormal amount of memory usage for Gw2.exe, so perhaps a VRAM leak?
I’ve been monitoring my temperatures to see if might be a hardware issue, and even tried underclocking the GPU and disabling turbo boost for my i7, but temperatures are no higher than normal and underclocking has not been helping any.
Pretty sure it’s a software issue, and unfortunately GW2 is the only game I play, or I guess that might be a good thing in ANet’s eyes, but I should probably start playing some of my other 3D games to see if it persists.
