First there's the obvious which is the massing of spyware and malware on your computer which can cause theinternet to slow down a large amount because of the bandwidth that gets eaten up by the spyware. I would suggest a good spyware detector to help you remove these and the best one that I know of (which is also free to download and use) is called 'Spybot: Search & Destroy', a simple search on google should get you to their web site where you can download it. :]
The other thing that can be the problem is the actual ISP. A lot of ISP's (especially those who are now offering the 8MB connection speed) have included a clause in their terms and conditions that allow them to 'strangle' the connection speeds at peak times to ensure 'fair usage', so that people on fast connection don't hog all the bandwidth. I think the best thing to do will be to check the terms and conditions of the new company you have joined to check for anything like this since it is something they seem to be doing a lot. Atleast in the UK they are I am uncertain about ISP's in the USA.
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