iMac skybox displaying with low resolution

with the following spec OSX 10.7.4 Unity 3.5.4f1

I have setup the sunny skybox within the game, and the outcome is not great, there are gradient lines throughout the entire texture (not seams). I have reimported the files, create new project, the usual routine and yet the problem persists. Is this normal for an iMac? Or were the skybox this poor from the start?

Thank you;