You on dsl or cable? What's your speed? You probably have a symetrical connection, meaning saturating your upload speed will kill off your download speed. Seems like you have a 128 Kb upload cap. 128/8 = 16 KB/s.......
I have a cable 3 Mb download/256 Kb upload, if I upload anymore than say 20+ KB/s, my download speed will plummet.