I have this problem as well. I think it's more an ISP issue as I have had two diff. high speed access accounts while using Xolox and it didn't do that on the other account I had with another company. (Wish I'd kept them but they were just too expensive.) The speeds were the same on both accounts but the one I have now seems to choke the download speed when uploading to the max.

I have it working pretty well now by setting the upload speed in Xolox to 6. Try that.