Have you tried running some speed tests at testmy.net?
This is a copy/paste of a reply I just did for another person having speed problems, but it would be the same for you...
Have you performed any speed tests at http://testmy.net/? If not, the Official reps on here will need some speed tests to go forward.
Create an account at testmy.net if you haven't already. While signed into that account, and using a single device connected directly to the modem with a LAN cable, NO ROUTER, perform a batch of five download tests using the manual 12MB test size. Space each test at least five minutes apart. If possible, run a batch of tests a few times a day, preferably in the morning, afternoon and evening, if you are able.
After you've run some tests, post your Results page URL so the tests can be looked at. It will look like the following, but with your testmy account user name where it says yourusername.