Did the restart help?  Sometimes i notice that zooming with the scroll wheel gets "stuck" or really slow.  I just hit zoom extents and things get moving again.  A 3DConnexion mouse is another really cool option for moving around in 3D space.  It works beautifully with SU.

also, make sure that when you zoom in or zoom out, you should have the cursor hovered over an object in the model. Sometimes if the cursor is out in white space the zoom tool will not work properly. sometimes if you end up inside a wall, or if you zoom in so far that you're floating in white space, the best think is to use Zoom Extents. There's also a 'View Previous' button that you can click (the icon is a magnifying glass with an over layed undo arrow.)

-mike