Adobe Premiere Pro - Audio goes out of sync after 40 mins of video

[Agent Sam Stanley]

Hello,

This has never happened to me before. So I'm editing a mp4 file, everything is going swell, then around 40 mins in the video the audio starts going out of sync. It starts in one of the scenes, one of the lines of dialogue goes silent, and then the audio desyncs. I've seen tutorials for when the entire audio is out of sync, but in this case it's only the second half of the film. I checked the original file and the audio is fine, the desync only happens in Premiere. 
Somebody said I should create a copy of the original file, then change the extension to mov and then import the mov file and that should fix the problem. Tried that and the problem continues. Don't know what else to do. I'm in the middle of a very long project, can't start all over again. 
Please help!

[Agent Sam Stanley]

So, an update:

Someone suggested that I should reencode the original file converting the frame rate to Constant with Handbrake and that should fix the issue. Tried that and the audio starts slightly out of sync (barely noticeable) and then around 10 mins same thing happens, one line of dialogue goes silent and audio desyncs. Again, checked the original file and the audio is fine, desync only happens in Premiere. 
Any suggestions please?

[ThrowgnCpr]

Please give more information about each of the files, the original, and any derivatives you are using in your workflow.  mediainfo output with framerates will be helpful for diagnosing the problem.

[ThrowgnCpr]

sigh. really?

User Agent Sam Stanley has been banned for 1 month for clearly using pirated material. Since this offense is by a new editor with no history of past problems, we're being generous by opening the doors back up in a month.

So next month, use a legal source, and you likely won't have any problems with frame rates. Note, the academy, and fanedit.org will not accept the Blair Witch project for review/submission to IFDb, and any edits that the editor does contribute will be looked at very carefully.

Remember kids:

YOU MUST OWN THE SOURCE

WE DO NOT CONDONE PIRACY