One of the first things that I suggest is that you try some of the speed tests available off of
http://www.dslreports.com , They have mentioned that time of day and internal network traffic/traffic on that site may cause inconsistencies in speed reporting.
I know that if you are using a router, you do need to tweak your system to use an mtu of 1492 or else you can't use their e-mail server so that may be part of the issue(
http://www.linksys.com/support/support.asp?spid=87). (you can change this by going to our tools and downloading/using dr.tcp off of dsl reports webpage).
Some other things that I recommend are just doing some basic things to get your pc into shape, defrag, scan for viruses/spyware and remove unneeded programs from startup.
I should also mention that your network settings should be explicitely set to the speed of your network (not autodetect), and you should only run the network protocols needed (mostly tcp/ip and maybe netbeui for peer to peer networking).
Hope some of these suggestions help.