Another thing you might want to check is a hidden splice within the wall or ceiling. I've run into this too many times to mention. Sometimes they are in a box and it's covered up with panelling or drywall, other times it's just an in-line splice with no junction box.
Sometimes in a case like this, it may cost the owner more to have you troubleshoot it than to run some new wires in the crawlspace, basement or attic (if they are accessible) between the affected receptacles.
