You are going to hate me for asking this, but are you certain you have the latest drivers? The actual AMD website would be the best place to get them....
If that doesn't solve the issue or you already have the latest drivers, there may be a conflict with certain settings...occasionally players have found that deleting everything in the My Games directory that is related to sins has fixed issues, though that was more a good troubleshooting thing for Diplomacy/Trinity...
If that doesn't work, I can't suggest anything that is quick and easy...there could be a simple setting somewhere that would fix the issue, but I'm not an expert on graphics issues...
Since you are having a problem with another game, the issue may not even be not Sins itself...could be an issue with the C++ Redistributables or DirectX...reinstalling both games (make sure to delete all content) may trigger some changes to those assets (or reinstallations) and fix the issue...
If reinstalling the games themselves doesn't work, you could try uninstalling the C++ redistributables themselves, then reinstall the games (which would trigger a new install of the C++ packages)..
