Disappearing cube face

The problem: Someone I know has created a rather huge map and they're beginning to
encounter issues with walls just vanishing.

He wrote to me the following:
______________________
As you can see in this screenshot the part over the door opening is
missing. However, it's not the cube that missing it is only one face of
the cube.

cs.nmu.edu/vnmu/temp/wrong.png

(Red circle around the area in question.)

In this screenshot you can see that the back side of that same cube is
perfectly solid and there.

cs.nmu.edu/vnmu/temp/screenshot2.png

I'm using Quark with map2diff_plus as map2diff would just error on me.

In the quark editor the cube in question is fine it's has all it's
faces and all the faces are textured with the same texture.

Thus far, I have been able to fix the errant cube in one of a few ways:

- Some time simply resizing the cube saving then resizing it back and
saving again will fix it upon recompile.

- Other times simpling retexturing the cube will fix it upon recompile.

- Lastly should nothing else work, I just delete the cube and make a new
one.

This really wouldn't be so much of a problem if it were only a matter
of a few errant cubes but the deal is the cubes needing fixing jump
around. I'll fix 2 or 3 and a new 2 or 3 possible more will appear in a
different spot in the map.

I'm really @ a loss as to how to possable fix this.

Thus far the only real idea I've had is making the headers (cubes above
the doors and windows etc..) into detail polys. thinking that maybe the
compiler need some help in defining the faces since the map was getting
so big. however this didn't help with the problem.
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That's what he wrote me. Thanks in advance for any help!