If you face an issue that your dwg files disappear in a section or elevation view when changing the zoom level, you can try to manipulate the far clipping settings to bring them back.
Often you can fix the issue by resetting the "Far Clip Offset" parameter on the view to a higher ( or highest possible) value or by setting the far clipping setting to "No Clip".

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I posted this fix a while ago at:
http://whatrevitwants.blogspot.com/2011/10/upgrading-rvt-to-2012-makes-linked.html
Posted by: Whatrevitwants.blogspot.com | November 14, 2011 at 09:32 PM