Hi, I had exactly the same problem. I had my broadband since October last year and since november/december my speed had dropped sigificantly to 320-340kbps. I had mucked about alot with the connection, eg, used speed twqeaks, patches etc over the months.
so about 2 weeks ago, I did a complete reinstall of my OS and amazingly, my connection has now gone up to 460-470Kbps average.
Also if you use an extension, then this can cuase the weak connection. I was using a 15M extension and now I have cut it down to about 6M by routing the cable through my chimney.
This didn't make any difference at first, but when I reinstalled my OS, the speed went up, so maybe it had some effect.
all I can say is if your using an extension, then try and shorten it if possible and if nothing seems to work, try a reinstall.
It worked for me.