I’ve had it a few times previously with other models but nothing like as bad as this. I need to send it to a client so really hope some kind person can help
I thought it might be a graphic card issue with my PC but I downloaded the model onto my iPhone and it views exactly the same on that too.
That’s not section cuts you are seeing. That’s called Clipping. Set the camera to Perspective and hit Zoom Extents. Leave the camera set to Perspective while you are modeling and reserve Parallel Projection for output.
In the file you uploaded, switching the camera to Perspective shows that the camer is inside the model. This is the cause of the clipping.