Sounds like possibly too much heat and light through window. Mine do best with medium light/ average room temps (about 74 degrees F in summer with airconditioning) and foliage is a deeper green which I prefer. Mine too though are now losing a few older leaves...it's just natural though, as new growth appears, for the older leaves to yellow. I just run my fingers through the branches to remove dying foliage.

A monthly (or as often as can be managed) shower of complete plant helps keep dust-free, pest-free too. I don't use fertilizer but I've read that if used you also need to flood soil on a regular basis to avoid buildup in soil. If it's just the older leaves and new growth is still appearing, I wouldn't worry. Just move back from window or to side to see if that helps.

One other thought: in winter resting-mode less water is needed than when new growth is in full swing in spring/summer. Good luck, hope some of these thoughts help. josh