We can't offer you any help without more information. Please post the following info:
1) Make and model of your modem.
2) Upstream and downstream signal levels
(click the link.) We are interested in your power levels (dBmV) and your Signal to Noise Ratio (SNR).
3) Have you talked to Comcast or had a tech come out? What were the results?
4) How long have you had the problem?
5) Is it constant or intermittent?If you are having speed issues:
A) Please connect your computer directly to the modem via Ethernet (no router, no wireless, no USB)
B) Run ShaperProbe
C) Run a Line Quality Test
Please post your results and let us know to what speed tier you subscribe.