Are all the leaves mottled or just the older ones? I can't explain any on the newer leaves. The older ones were treated for scale and spidermite some time back and some didn't react too well to the combo I used (Orthene, imidacloprid, Distance, and Avid plus a spreader-sticker). I can replace it if it's on the young leaves too and you think it's a virus. Try a well balanced fertilizer every week (I know you already do) and reduce the light. It takes good light but not full sun. The only insect I can think of that might cause some mottling is boisduval scale. But that would kill the leaf and p-bulb too.
Gary