render loss

I created a movie with a captions track. Now I want to render it because without it, the timeline stutters. At first, everything red turns blue, and after a while (about halfway through), the blue turns red again. So, the rendering is gone again. Does anyone have any idea what this could be? My new Mac is a Mac mini 4 Pro with 48gb

You could try lowering the render speed to enable the computer to keep up. You will find it under the File tab in the Render Settings.

He's referring to Cache here, Charles.

I created a movie with a captions track. Now I want to render it because without it, the timeline stutters. At first, everything red turns blue, and after a while (about halfway through), the blue turns red again. So, the rendering is gone again. Does anyone have any idea what this could be? My new Mac is a Mac mini 4 Pro with 48gb

This doesn't make any sense. What camera source format are you using? What are the timeline specs (resolution and framerate)? A Mini M4 Pro should be able to handle almost anything, unless you're doing a ton of node processing and/or noise reduction.

You should also check the Cache Settings, which are under the Playback menu. This will affect playback performance, as will the prefs in User -> Playback -> Performance mode. For most people, I think Automatic should work OK, but there might be situations where you need to change it and see if that improves playback.