Graphics Card shutting on and off

OS: Windows 7 Professional x64
CPU: AMD Phenom(tm) 9750 Quad-Core Processor (4 CPUs), ~2.4GHz
Motherboard: ASRock A785GM-LE
Memory: 8192MB RAM
Graphics Card: AMD Radeon HD 7700 Series
Sound Card: Speakers (Logitech USB Headset)
Monitor(s) Displays: Coby LED 19"
Screen Resolution: 1366x768
Keyboard: USB Keyboard
Mouse: USB Mouse
PSU: Corsair TX650
Case: Antec Three Hundred Mid Tower ATX
Cooling: IDK
Internet Speed: 11 mbps

System Manufacturer/Model Number: CGR Watsonator Custom rig
OS: Genuine Windows 7 Home Premium x64
CPU: AMG Athlon II X2 255 3.1/2MB/2000MHz 45nm AM3
Motherboard: Gigabyte GA-MA78LMT-S2 AM3, 760G, Ultra Durable 3, M-ATX
Memory: 2X 4GB Kingston DDR3 1066MHz @ 1600MHz Dual Channel
Graphics Card: HIS HD Radeon 7770 GHz Edition iCooler Core: 1000MHz
Sound Card: Genius Sound Maker Value 5.1
Monitor(s) Displays: Dell E2311H DVI/VGA 5ms response, 1080p
Screen Resolution: 1920x1080 @ 60Hz
Keyboard: Logitech K120 Wired Keyboard
Mouse: Logitech K120 Wired Mouse
PSU: Cooler Master eXtreme Power Plus 700w
Case: Lian-Li PC-6010 With Side Window
Cooling: Aywun A4 80mm fan
Hard Drives: Western Digital Caviar Blue 500GB 16MB Cache 7200rpm SATA III
Internet Speed: 802.11N 300Mbps+ ADSL2+ Max 12.12Mbps
Other Info: Speakers: Logitech LS21 2.1 Satellites + Sub woofer AMD Catalyst 12.11 BETA Linksys x2000 Dual Band Wireless N 300Mbps ADSL2+ LiteON CD/DVD Player GAME PERFORMANCE: Battlefield 3: 41 FPS Ultra 1x AA @ 1980x1080 Mass Effect 2 and 3: 60 FPS Highest 4x AA @ 1920x1080 Counter Strike Source (and other Valve games): 300 FPS (max FPS in game) Highest settings 8x AA 16x AF @ 1920x1080!!!