The problems you are experiencing are most likely due to
Linework is not coplanar (some linework either sloping in the z axis or at totally different elevations) and / or
You have dirty CAD linework - Overshoots, Undershoots, Lines not Intersecting correctly, Slivers, Double linework and other such Junk / Spaghetti.
Try flattening all linework in your CAD program - if you have access to Autocad - the Command is FLATTEN if you have Express Tools loaded - then you'll KNOW its in 2D.
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Clean up the CAD Linework as much as possible if it's at all feasible.
Alternatives - Visit Smustard:
http://www.smustard.com/scripts/
Download 2 scripts - Flatten and Make Faces.
Once Data Flattened - Run Make Faces Script.
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Hope this helps.
Howard L'
