I am using a three monitor setup and have noticed that when zooming into specific areas in certain large DGN files the elements in that area seem to disappear. If I zoom back out they may re-display but often disappear again when zooming in.
The issue is most likely graphics card related, but other factors may include the viewing angle (orthogonal vs. isometric) and the complexity of the geometry. If you find this happens consistently the following steps might help:
1. From the active view window, open View Attributes
2. Enable the Clip Back and Clip Front options
3. Fit the view
4. Try zooming in again