If it were my plant I would re-pot it. I imagine it must be root bound. It looks very healthy, cut away the dead leaves and re-pot it in the spring and it should be good to go. I'm sure others will come alone with more advice for you :>)
Christine

Are you watering it with tap? If you are staet wateeing it with spring bottled water.it might also be root bound. Hope this helps :)