Just an idea, may be far from what is really going on.
Some times when I'm downloading something very fast (considering my connection speed), and I have my amule connected, upload speed decreases. It doesn't matter if I'm downloading something from an ftp, or amule, whatever. So I guess upload speed decreases just because my pc can handle connections properly, or due to my router, my ISP.... or it is the way it's suppused to be.
May be it is your situation.