Another tip: The status page in your modem should have statistics about the quality of the signal it is receiving. Different modems have different pages.
The main stats you want to look for are:
Used Line/ATM/Bit Rate/Speed: This should be the speed at which you're provisioned. Meaning it should be pretty close to the advertised speed, if not higher.
Noise Margin: Should be at least 6db both directions.
Attentuation: Should be less than 75db both directions.
Issues with these values indicate a trouble from the modem back but not necessarily a network or provisioning error. To isolate further, try a different phone cable (<15ft) and/or jack. To absolutely isolate your inside wiring, connect the modem direct to the NID and test. This is probably not realistic in most cases.
To clear your local network connect a single computer directly to the modem, bypassing any router you may have. Also, make sure software on the computer isn't causing the issue. Disable all P2P programs, servers, firewalls and ensure that your PC is virus free. Remember that a saturated upstream will wreck your downstream. Also, make sure you clear your browser's cache (temp internet files) before testing.