Playback Lag & Audio Cutting

Hi everyone,

I'm working on editing a film right now, and there is severe lag in the playback/preview window while editing -- but only for the files that pertain to this film. All of the files I use are .mp4, and I've had no trouble editing similar files in the past. In fact, whenever I load up another project with the same files from the same camera, they work fine, with no lag whatsoever. It is only these particular files. I've done most of what the internet and these forums have told me -- I've cleared and manually deleted cache files, used proxies, lowered the quality of the playback window, and pre-rendered the sequence. All of my sequence settings are the same as my other projects, and I'm mystified as to why it's happening.

Also, the audio is cutting in and out and random intervals; once again, this is only on these particular files and no others. Thank you for your help.