The most common causes for those kind of playback errors would be:
* Networking issues (if data delivery to MC stalls, then playback will stall or fail). But this would be ruled out if you tried local copies (which is why it was suggested).
* Video card or audio device driver issues (which could be a new issue, even if the driver didn't change, with a newer Windows Update or version of MC). You may want to check for updates for your video card and audio device drivers, or possibly roll back to an earlier one (especially if you recently updated something).
* Security software interference. AV and other security suites can change their behavior with definitions updates, and do all sorts of crazy, unexpected, and mean and nasty things to interfere with MC accessing files on disk (which would seem, otherwise, to be the basic point of an Operating System. Anyway, AV stuff gets in the way, and it changes sometimes in unexpected ways.
* File corruption on disk. Check to ensure the files in question play properly in other players (MPC-HC is a good choice since it will also use the LAV filters like MC, and maybe VLC player).
JRiver did recently include a patch for a bug related to networking stalls in the beta build, but I don't know that it would manifest like this. I'm sure that version will be out relatively soon, but it is absolutely worth checking the other stuff above as well.