Same issue here. I'm running a 2012 iMac with 680mx and an aja IO XT. I've had this issue with all kinds of media, but it's especially bad with Sony XAVC. In addition to the playback stopping, I'm having dropped frames and drifting audio sync. I tried updating Cuda drivers, removed 3rd party plug ins, reset preferences and spent a good amount of time on the phone with Adobe and Aja but have had no success. Also, the system has become wildly unstable and will crash ("serious error"), sometimes requiring a hard restart of the computer.
I've tried running the same projects and hardware with my other mac, a retina mac book pro and have the same issues.
Bob Pierce