I suppose you have the latest service pack loaded? Do you have the autosave on or off? I have turned the autosave off and tweaked my quicksave to have run the audit before saving to fix errors.
I am working with several surfaces, trying to paste them together. I have to do a couple of pastes and then save or i'll get and error message, not fatal, but my drawing is no longer editable. I have to close and reopen.