MPlayerX on OS X suddenly doesn't interpolate video anymore? (fullscreen = ugly pixelated output)
I had the same problem at my both devices, MacBook Air (mid 2011) and 5k Retina iMac (mid 2014) only after upgrading to El Capitan. I tried everything to solve the issue with MPlayerX (as it's my fav player and most used app) but no luck. Later i downgraded my MacBook Air to Yosemite and problem was solved.
Now I don't think it's a OS X related problem because other players are running smoothly so I guess it's high time for MPlayerX to release a new version.