Sorry I have no solution. I just wanted to point out a similar situation I have with hope that someone will know what is going on for both of us.
We use Civil 3D 2010. I have a road design drawing where everything is completely normal for a little while. After a while, things I erase or trim do not disappear and lines that I extend do not appear. Regen all doen't help.
I save, close and reopen the drawing the changes where applied and I can continue to work on the dwg 'normally'...for a while.
And yes, like Josh, I have purged, -purged, audited and recovered the drawing many times. No Errors in this or any xrefs.
As far as I can see, It's not a memory issue. I am running Win 7 64bit (Enterprise) with 8GB and a Quad Core with 256Mb Video.