You are using a preview version of the drivers and these are mostly meant for testing, they might not be finalized or have bugs etc. Maybe that's why you are having problems.
I have a much older card than yours (Ati 4870 1GB) and it's supported fine by the latest
official release of the drivers, which is the 11.12 version. I really doubt they would stop supporting a relatively new card when much older ones are working on the same drivers.
Instead of using an older version of the simulator, it would be better to use an older (just one month older) and more stable version of the drivers. Try uninstalling your 12.1 preview drivers, rebooting your PC and then installing the 11.12 drivers instead.
Hope it helps
