Quicktime and Mplayerx not playing shots smoothly

Since a few weeks, I am observing that my players MplayerX or Quicktime won't play smoothly my shots. When the clip starts it will take a few seconds in very low speed, then it plays, then it get stucked again. This happens either with mts files or with transcoded mov files. This happens when I am playing the clips from the finder. Once they are imported into FCPX, everything is smooth. But sometimes you need to watch the footage inside the finder. I never had this phenomena before. Thats why I think its strange. I tried already to trash and reinstall the players. Any ideas about this? I am running OSX 10.9.5 on a 2.0Ghz i7 8GB Macbook Air 11, which works brilliantly even with huge projects in FCPX. Any suggestions or ideas would be appreciated. Thanks a lot.