It sounds like you may have accidentally enabled Transform Snapping (⇧ Shift↹ Tab).
By default transform snapping will snap objects to fixed increments as they are being transformed.
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.
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 Shift+Tab: