Already tried to use a clean configuration file (e.g. renaming your q3config.cfg to something else)?
(DO NOT LINK) h t t p s : / / openarena . wikia . com/wiki/FAQ#Configuration_files_and_automatic_downloaded_files_path
I did try this, yes, by downloading a new copy of OA
It fixed the lines down the screen but the old problem of the screen dimming on startup came back.
I'd say there is some sort of problem with your graphic card…The fact that on 10.7 an application can't (really) change resolution anymore may prevent it from happening…
Questions: if you run the game in a window the problem still occurs? Are you still on 10.7 or by any chance you've updated to 10.9?
Now that you mention that, I think it is the problem. I had another play around with the settings and I found that changing the resolution down from my native 1280x800 resolution fixed the lines problem, but as above; the screen dimming problem is back.
If I run the game windowed, the same problems occur. And I am still on 10.7.5 as that is the furthest my model can update.
I have read some other threads detailing the dimming problem and possible fixes but I don't quite understand what to do.
Thanks for all the help anyway!
[EDIT] After trying some more things, I have found that the lines down the screen appear on startup in both windowed and fullscreen. They go away when I 'refresh' the resolution settings by changing and changing them back. When the lines go away, the screen dims, which can also be fixed by 'refreshing' the brightness settings.
So basically, the game is still playable and works fine except for the display settings refresh that I have to do on startup. I guess this is bug with the game, or something wrong with my Mac.