How do I fix this. Perspective then Extents Zoom takes me to view of model but it has changed to Parallel Projection and this makes model unstable.
Have you changed the default viewing angle with the Zoom Tool?
The viewing angle can be adjusted from 1 degree (super telephoto) to 120 degrees (wide angle).
The angle can also be specified as with a camera lens by adding mm (for millimetres) as for a camera lens. In this mode, the number is as you find on camera lenses and can go from 10 mm (wide angle lens) tp 2063 mm (super telephoto).
When set to very narrow angle or very long focal length, perspective view becomes similar to parallel view.