1. Here's why
Flickering shadows are usually caused by the fact that the models are too far away from the center of the Scene (0,0,0). This is a known technical limitation as graphics cards only have 8 digits for positional values. The further away from 0,0,0 your models are, the more likely it is that their shadows will flicker.
The center of the Scene is marked by the red, green and blue axes:
1.1: To prevent the shadows and surfaces flickering, you should move the models closer to 0,0,0.
The easiest way to do that is to add the models to a Group:
1.2: Then move the Group to 0,0,0:
Knowledge Base: How do you type in position coordinates for an object?