Hi Matt,
Please mention the exact version of Premiere Pro. Assuming you are using 2014 version, please try some steps mentioned below.
1. Clean the media cache files by navigating to /Users/User Name/Library/Application Support/Adobe/Common
2. Check the version of the drivers of your NVidia card. So, go to Apple Menu>System Preferences and check if you have a CUDA Preference pane if it is not there then go to the below link and update the Card to latest drivers.
CUDA Drivers for MAC Archive | NVIDIA
3. Also, please mention if you have any third party plugins used in the project. If yes, then try to update those Plugins or copy and paste those plugins to some other location and Trash them from MediaCore of Premiere Pro. The location of third party plugins is mentioned below.
Startup Drive or MAC HD /Library/Application Support/Adobe/Common/Plug-ins/7.0
Hope it helps.
Regards,
Vinay