For me part of the system hard locks after about 30-40 minutes. This happened in AC Unity too, not sure what is the cause. The display goes out, so monitor tells me there is no signal, there is a weird buzzing sound from audio, and the GPU fan goes max speed. Sometimes it reboots itself after 5 seconds or so, sometimes I have to manually reboot it. Other games, CS GO, DOTA 2, BF4 gives no issues. I replace the cooling paste on my GPU every 6 months, and just did again this morning. But it still crashed. Also, temperatures are well below limits at time or before crash. Max CPU temperature is about 65-70C, and GPU is 65-70C. Overclocks are mild too, 2500K@4.2GHz, and GTX 680 at +10Mhz, +150MHz.