I have that issue also now for several months. My Old GFX was a Radeon HD 5870 v2 with 1 GC DDR5 G-RAM and i have now since a week a R9 280x with 3GB DDR5 G-RAM. And still i get random crashes when i port out of crowded areas after about 4h or so and FPS drops and then sudden invisible Monsters.

First i tought is was my old Grafic Card but now where i have a brand new one and still the same issue i know it is not my PC.

Speccs are:

Windows 7 Ulitame 64bit
ADM Phaenom 2 x6 with 3.2Ghz per core
4GB DDR3 RAM
ADM ATI Radeon R9 280x with Catalyst Version 14.4 Drivers