Audio cuts out random

Phone footage is some of the most challenging and problematic to work with. That's due to the combo of HEVC and Variable Framerate.

HEVC is the most complex video codec you can be working with right now. It's really great at using temporal compression to get things into a small file size and maintain quality, but the advanced algorithms used to do that are exactly what makes it incredibly difficult to decode (play). Your computer is doing complex processing to figure out what each frame of HEVC looks like since each image has to built from a large group of the surrounding frames. 

Combine that with Variable Framerate (VFR), which is like poison for editing software. Editing software like Premiere likes a constant framerate since it's using timecode to precisely keep track of where every frame and every edit decision you make lives. When the framerate is constantly fluctuating up and down throughout your clip it causes all kinds of issues from long load times and media pending issues, longer than usual encoding times, audio will slowly drift out of sync, frame substition errors, and just a host of other errors and unpredictable behavior.  

The combination of those two things make for just about the worst editing experience you can have right now. Transcoding those clips ahead of editing to correct the VFR and get it into a more manageable video codec can completely change your editing experience. If it were just a bad editing codec like H264 (AVC) or H265 (HEVC) then I'd say to make proxies, but you can't make proxies out of clips with VFR because the proxies won't line up.  

In general, the majority of issues people have with performance, playback, etc., all come down to the media you use, not as much your hardware or editing software, so the more you learn about that technical side of things the more you'll understand about why certain things happen the way they do and how you can resolve those issues and have a smooth and stable editing experience.

Some resources:

Information on video codecs in general: https://blog.frame.io/2017/02/15/choose-the-right-codec/ 

Information about VFR: https://www.reddit.com/r/VideoEditing/wiki/faq/vfr/

Also worth trying would be clearing the media cache: https://helpx.adobe.com/premiere-pro/kb/clear-cache.html

And another couple of things that can help or can be related to those audio playback issues:

- Making sure you have enough I/O bandwidth on your storage, ports, etc. so that you aren't saturating the speed with data. Since you're working with very compressed source media it's unlikely that this is the issue, but it can result in similar behavior.

- Moving your audio cache to a faster drive like an SSD or NVME