i get the same problem, though its rare like 1 in 30 or more, locks up pc after exiting a game, defiance, ut3 were my last ones. pc specs:
PC RIG Number 4 Based on Motherboard:
Operating System: Windows 7 Home Premium 64bit +HUD Machine(Green) Theme by Mr.Grim
Motherboard: Asus P7P55D-EVO
CPU: Intel i7 860(B1 Revision) @ 3.516ghz, 21x Multiplier @ 1.184v, HT ON, + CoolerMaster Hyper 212 Plus CPU Cooler + 2 CM Silent 120mm Fans @ 7v
Memory: 8gb(4x2gb) G.Skill Ripjaws DDR3 @ 2009mhz 2:12 FSB

RAM 9-9-9-27-2T Timings @ 1.65v
GPU: 2x Sapphire ATi Radeon HD 7950 Boost 3gb's in Crossfire @ 925/1250
Sound: VIA® VT1828 Onboard
Capture Card: AverMedia CaptureHD H727
PSU: Corsair HX1050 +PSU Silicon Spacer
HDD: Total 11tb, Samsung HD103SJ 1tb(Main) 7200RPM, 2x Western Digital WD20EARS 2tb 5400RPM(Backups), 2x Seagate Barracuda ST3000DM001 3tb 7200RPM(Anime Drives)
Optical Drive: LG GGC-H20L Blu Ray + DVD Burner
Case: Coolermaster HAF 932 with 8x 120mm Xigmatek Green Led Fans @ 7v +8 Silicon spacers with 4x M5 Screws Per Fan
Monitor: BenQ V2420H 24" LED TN Panel 1920x1080 60hz
Speakers: Panasonic SA-AK640 3-Way Bi-Amp Bi-Wired 540w Hifi + Logitech Z-5500 5.1
Headphones: Audio-Technica ATH-M50S
Keyboard: CoolerMaster Storm Trigger Cherry Red Mechanical Keyboard With Black 50A-R O-rings
Mouse: Razer Deathadder 4G 2013 Edition @400 dpi
Mousepad: Razer Goliathus Fragged Alpha Speed Refresh
Catalyst 13.1
i believe its a windows 7 OS problem.