Clips Turning Black

I have been using a free trial of Premiere Pro (on my macbook pro) for a University project, but due to it crashing every few days my project is still not done and the free trial has ended. 

While using Premiere Pro, I have been getting bombarded with multiple error messages. When this happens, a few clips freeze on a random frame (or turn black) and don't play. I don't think it's an issue with rendering, as the clips also stop playing outside of the timeline. Although the video clips in question play completely fine outside of Premiere Pro, when I have imported the same videos again, the same thing happens to them. After importing them again but from a different source, I managed to replace the faulty clips and carry on. However, the same thing has happened again. Does anyone know what might be the issue? 

Thanks