I have the same problem opening some AutoCAD Mechanical drawings. I have not figuired out how to solve this either. I always do a PURGE after recover and save but then one day out of the blue it comes back. I have noticed a consistancy with them though. The only drawings out all my drawings that do this are ones that have blocks in them that were downloaded originally from the Allen Bradley/Rockwell website. I don't know what anyone can make of that but since its consistant maybe its one of those blocks. Everything else in the drawing I drew up myself. And those blocks are not straight from their website. I download, open them up and explode them so I can change them to the layer that we use for our blocks. Then I reblock them and put them in our Library.
I posted some code a while back that may help
http://forums.autodesk.com/t5/AutoCAD-MEP/MEP-2009-open-drawing-recover/m-p/2233374#M20978