Okay, the next step would be to run some speed tests. Please head to http://testmy.net/, set up an account there, and run some tests while signed into that account.
Please make sure to run the tests with a single device connected directly to the modem, no router.
Please use the manual 12MB test size for download tests.
Please try to run a block of at least three tests, each test spaced at least five minutes from the last. If you are able, please run a block of three tests at various times of the day.
When you have performed a few tests, please post the results URL. This is done by clicking the "Results" tab on the upper right and pasting the URL from the results page into the post. It will look like the following, but with your user name where it says your name.
Creating the account and running the tests under that account are very important. It allows the tests to be viewed from a single location and the results can show a pattern.