It could be a number of things
Old/outdated/damaged cat5 cables
Your ISP throttling you because you use a lot of bandwidth
Your ISP throttling you to force you into paying them more money for the speed you should be getting
Or just a lot of traffic across the web clogging things up
My ISP pulled a bitch move like that recently
They didnt inform us that they stopped offering the internet package we were on, and was charging much less for the same speed
When we changed to the cheaper plan, our internet got just a little faster, but we are getting no where near what we are paying for (SBC global)