Vanishing walls

Vanishing walls: I am using model to draw a floor plan and then to proceeding to push pull the walls to the required height. As I orbit around the building the walls are vanishing and then partly returning, leaving me with an incomplete building.
Is this some bug in the programme ?
I cannot do any work whilst this is happening. I have started fresh and the same thing happens. .
Can anyone shed a light on how to cure this problem?

Clipping and missing faces | SketchUp Help

Thanks, easily fixed. It was the “zoom extents”. Another lesson learned!