Hi!
Happy to say I figured it out! Using macOS Activity Monitor, I saw that over time the memory usage of the experiment was ramping way up, and whenever it reached ~3GB, the screen froze. After troubleshooting I figured out that it wasn’t the movies that were causing the problem at all but the text stimulus I was using (as part of the experiment participants see a countdown indicating when the video will end). In the end, my issue was very similar to the one from this post: