I'm certainly not a knowitall, though I wish I was. However, I do have a couple of suggestions that may or may not work. You could go to tools - options - uploads and decrease the amount of bandwidth you allow for other users. Or you could go to downloads and see what percent of your bandwidth is devoted to your downloads. If those don't help, you could decrease the number of simultaneous downloads.
If all of this stuff doesn't work, it could be that the people you are downloading from don't have much bandwidth for people trying to download from them, so when other people try to download from the person you're downloading from, your speed will go down. I have no idea why your progress bar would go down. that's possible?