It sounds like you may have accidentally enabled *[Transform Snapping][1]*.

Transform snapping by default will snap objects to fixed increments as they are being transformed.

> **Transform snapping**:  
> The ability to snap Objects and Mesh element to various types of scene
> elements during a transformation is available by toggling the magnet
> icon (which will turn red) in the 3D view's header buttons.
> 
> **Increment:**  
>     Snap to grid points. When in Orthographic view, the snapping increment changes depending on zoom level.

You can disable it by clicking the magnet icon in *3D view > Header*:

![Enabled][2] ![Disabled][3]


  [1]: http://wiki.blender.org/index.php/Doc:2.6/Manual/3D_interaction/Transform_Control/Snap_to_Mesh#Transform_Snapping
  [2]: https://i.sstatic.net/A6zFc.png (Enabled)
  [3]: https://i.sstatic.net/6re11.png (Disabled)