Intermittent access/speed

For the last few weeks I've had intermittent access and speeds (0.5mbps-12mbps) with the preffered 50mb package.  This is with multiple computers, different routers, and swapped cables.  I only have a docsis 2.0 modem.  I've talked to others in the Phoenix area who have had similar issues.  Do you think I need to upgrade my modem, or has this been a consistent problem with others?