On MouseOver?

Hello there. I was wondering how could I implement a mouseover feature in my OpenGL App. I have already implemented a "RayGun" class that can "fire" at objects that are in the current viewing frustum. Previously I made it so that you needed to click in order to cast a ray. So I thought that If I wanted to do a MouseOver-like feature I would have needed to fire a ray every frame... but well it seems that the current scene that is displayed is lagging behind the position of the mouse. What I mean is that objects gets highlighted after several seconds that the mouse has passed over it, and other strange results. But FPS are still very High (they are capped at VSync - 85) Are there other ways to do it? And is there a simple openGL function to highlight the outline of a geometric object or something like that? (I need to display a sort of tool-tip over it and perhaps draw an outline around the object..) Thanks in advance.