Media Player in sync for 40 minutes, then out of sync

[wmp]

I'm running a Windows 7 Home Premium 64 bit, and for some movies, the video will start in-sync for about 40 minutes, then slowly get farther and farther out of sync, making watching the video un-watchable after another 10 minutes. But it does not do this with all video types. It also does not do this with Netflix, YouTube, or anything else online.

I'm running a GeForce GTX 660M, all drivers are up to date.

The files that work are: MP4,

Files that don't work are: MKV, AVI

Those are the only files I have tried. It's done this since I got this computer a couple months ago

Thanks for any help!

 

[wmp]

Oh, but when I restart my computer, I am able to watch another 40-ish minutes, restart, etc... It gets frustrating