When the FPS drop below 60 the player character movement is not smooth anymore but "jumps"
Here is the Lazy Foo's Frame Independent Movement tutorial:
maybe this helps http://www.gamedev.net/topic/651698-advice-to-lock-frame-rate/
Improve frame skipping code. Fixes #287
The new code takes into account that the non-graphic updates take some time, too and only when there is time in the current frame left the graphic is rendered. The game logic always runs at 60 updates per sec.