It sounds like you may have accidentally enabled *[Transform Snapping][1]* (<kbd>&#8679; Shift</kbd><kbd>&#8633; Tab</kbd>).

By default transform snapping 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*, or by pressing <kbd>Shift</kbd>+<kbd>Tab</kbd>:

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

For Version 2.8.

[![enter image description here][4]][4]


  [1]: https://docs.blender.org/manual/en/dev/editors/3dview/object/editing/transform/control/snap.html?#transform-snap
  [2]: https://i.sstatic.net/A6zFc.png (Enabled)
  [3]: https://i.sstatic.net/6re11.png (Disabled)
  [4]: https://i.sstatic.net/81wPq.png