I am having exactly the same problem,
I have been using the
thinkbroadband :: The UK's largest independent Broadband / ADSL comparison & review website to test the speed of up load, and it either starts of OK but when it gets to up uploading 40% of a .25 meg test file it stops for a period of a few seconds and then it starts transmitting in small batches giving a final test speeds as low as 5kbps, some times it stops and gives an arror message.
2 to 3 weeks ago I was able to tranmitt up load at 370kbps.
I have changed the hub, used another computer still the same.
At the moment BT wholesale has restarted my connection to see if this will bring the speed up, my down load speeds are getting back to normal , but up loading is still staying slow, and when I carry out a test the same happens as before.
If you get a solution please inform me, and I will do likewise.
One last thought, where are you located, I wonder if we are using the same exhange / equipment and we have a linked fault.
REGARDS
Dave Matthews