If I leave it for any time while I am working in another application and come back to it, the program refuses to play back. Shutting down the software and restarting doesn't work either, rebooting the machine seems to be the only fix. I am cutting on an i7 iMac with 32 Gb ram and using two thunderbolt equipped RAID drives. This isqsue shappens irrespective of whether the media is held on the external drive or on the internal HD?
I also sometimes have an issue with playback. Althout I press play/space nothing happens. Restarting Premiere helps. Very weird. I am using CS6 also with a i7 chip but on Win7 and with 8 GB.
Power Schemes, Screen Savers...????
I found a solution to another problem with PP maybe it will help you as well. Try to turn of any setting in you graphic card that has to do with OpenGL and see if that helps.
I am using PP CS6, Win7 and a nVidia Quadro1000M. Maybe it will also work on an iMac with ATI graphics.
For more information see this post: http://forums.adobe.com/message/5884237#5884237
Let me know if this helped.