atikmdag is a general c atch-all error that helps keep the driver from crashing the whole computer so you can diagnose things.
assuming you are not overclocking, temps are normal, ram is fully compatible, then you'll just have to try some stress testing to find the issues/s.
First try lowering your graphics settings to see if it plays better. also, what are your cpu/gpu temps?
How is your PSU? This sysmptom can come from lack of power your your system.
GigaByte GA-970A-UD3 | AMD FX-8120 | 16GB (4 x 4GB) G. Skill DDR3-1333 C7 | MSI 7750 OC | 1TB Seagate 7200.11 | Dual boot Windows 7 Ult x64/Ubuntu 10.10 x64