That's a very nice, big mature plant. How long have you had it, Jason?
I'm wondering if maybe it's time to give it a bigger pot, and some fresh potting mix? Might be worthwhile taking a look at the roots - just lay it on its side on some newspaper, and slip it out of the pot. IF you see a lot of roots crowded in there, right out to the walls of the pot, and going round and round, you need to re-pot it. If not, just rinsing some of the old soil off the root ball and adding fresh potting mix might give it a healthy boost.
Fertilizer is probably a very good idea - an indoor plant has no other source for nutrients than what you supply. Potting mix might have some fertilizer in it, but usually that is used up in the first few months. You should also fertilize and water a bit more in the summer, and less in the winter because of the variation in light from the window. The room probably gets cooler at night in winter, too. So indoor plants do experience seasonal changes.
As Tara said, a mature plant is also expected to lose older leaves as it grows new ones. So you shouldn't worry too much as long as the spotting and yellow blotches don't increase.
