Objects Disappearing in AutoCAD 2014

I have a classroom full of compters and every year when we work on 3D modeling, the majority of the class has problems.  We are creating pretty simple 3D shapes with AutoCAD 2014.  

Probelms:

1.  Shapes will disappear and reappear when zooming.

2.  Students will save their project and come in the next day and it will be visible in the "preview window" but when you open the file it is not there.

3.  If you change from shaded to wireframe, the shape usually becomes visible. We have been making objects shaded but changing to realistic or conceptual doesn't seem to help either.

I'm guessing it is a problem with computer requirements but is there anything with settings that I can try to change?

I belive my tech dept. put a better graphics card in one of the computers to see if that helped but did not. 

My computer is:

Dell Optiplex 780

Windows 7

4 GB Ram (3.87 GB usable)

64 Bit

Intel(R) Core (TM)2 Quad CPU Q8400 @ 2.66 GHz 

Service Pack 1

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