every once in a while, for no reason I can determine, I'll try to drag an object, and instead of moving around in the X/Y coordinate system, it will scale (get bigger or smaller) or rotate.

When this happens, the centerpoint icon for the object changes to either the rotation or scaling version, and the only way to get it to move again is deselect and reselect the object (sometimes several times) until it starts moving on a click-and-drag again.