Did you try the etransmit as Kirk suggested? I have never tried that, but it's worth a shot.
Another thing to test; since you said the export normally works, but not on certain drawings. Copy/paste everything from the drawing you need into a new drawing from your template. Save, audit. Try to export to acad from there.
Win 7 Pro, 32 bit; Intel Core i5 @ 2.80GHz; 4GB RAM—Civil 3D 2008 & 2011
Credit where credit is due! Give kudos or accept as solution whenever you can.