outside of the aphid comment, it sounds alot like your seeing sun damage. was the palm recently purchased/moved from a greenhouse? It would help to see a picture of what your talking about

I have found that mine have the little brown lines and I believe it to be part og the fiber of the leaves,not necessarily aphids.As for the dark spots.. are they at the edge of the leaves or randomly placed.

I am a grower for a nursery when you get insects you usally get fungus try spraying with insecticide with a copper fungicide and watch the new growth that should help let me know how it it is doing