Sometimes one can get a rust spot in the cable housing which can cause problems, especially if there is a low spot in the routing. Was the old cable badly rusted at some point? Little nicks in the plastic covering for the cable housing can also spell it's demise.

Grease can sometimes help.

Nonetheless, the replacement housing is cheap, as long as you can get a close enough match, and that is probably what I'd do.

Your mechanic will probably also recommend a new cable housing as he will also lack patience with trying to force cables through a housing that isn't good.