A few things.
Have you updated your graphic card driver - it is the latest/newest available from AMD's website?
Do any other games freeze?
Is it always half an hour?
And do you know if your CPU/GPU is overheating?
Yes i have the latest driver for my GPU,no it is the only game that freeze,s on me,yes mostly half an hour to an hour then it the computer locks up,i have also checked the GPU/CPU the the tempature seems fine.my brother has near the exact same computer as mine and his doesnt lock up,bloody strange