What did you set the TDR to? The delay should be by default at 10sec I think. Did you try higher? Any difference if you do set it to say 15?
This link was pretty comprehensive:
http://support.amd.com/en-us/kb-articles/Pages/737-27116RadeonSeries-ATIKMDAGhasstoppedrespondingerrormessages.aspxf
and if you reinstalled windows, and tried everything on it (tested your cpu-Prime95, gpu-Furmark, memory-Memtest), and only gpu fails, then as they list it themselves, replacing the hardware is the last option.
So if I were you, I'd take that list, literally check off each point and write to them what you've tried, including results of all the tests, reinstalls and everything, attach test outputs if necessary and then ask whether you should RMA or not because at this point there's nothing left to test.