Seems to be happening more since the patch, had occasional ones pre 1.19.

Location : Random

Action : walking / standing around, seems totally random
(Though talking to Npc's or entering Quest instances seem to be the most common but I don't think its related..)

Time : None specific
Time : In game : Random

OS : Win7 64Bit
CPU : i7 2600k @4.4
GPU : 570 @ 850/1700/2000 (FPS cap @30 to test difference)
IP: VM(uk) 20mb cable