If your using TCP optimizer or dr TCP (my self i have both) i would check out the tweaks you have done on this web site http://www.dslreports.com/tweaks
then do a speed test. I used TCP optimizer but after running the tweak test on dsl reports it suggested changing the rwin. after a few changes and reboots I finally got that everything looked good. my speed went from 4 to 6.5 and it seems to stay around that .. my isp is pipex and i am supposed to get speeds up to 8 meg. hope this helps