Are you using the manual 12MB test size for download tests and the manual 2MB size for upload tests (normally it's 2MB, though Chris asked for 3MB)? Also, you should be using a single device connected directly to the modem with a LAN cable, NO ROUTER, when running the tests.
It would be best for you to create an account at testmy.net and run the tests while signed into that account. This will keep all of the results in a centralized area that can easily be looked at and which can show patterns. When you've run some tests, you can post your results URL by clicking on "Results" on the upper right and then copy/paste the URL in the address bar. It will look like the following, but with YOUR user name where it says "yourusername".