Buggered if I know. I've done my fair share of travel in 738s, albeit mostly within Oz, but I've never noticed the symptoms that you refer to. However, it's possible that you've answered the question yourself because of the effects of pressurisation changes.
Maybe someone else has a better idea? I suspect there's been enough problems over the years that all decent operators take all the now standard precautions with airframes that have logged a lot of pressurisation cycles.