DECREASING DOWNLOAD SPEEDS

I moved here last month and signed up for the Ultimate Internet package but my speeds have greatly decreased from 20-30 mbps when it was initially set up, to 2-5 mbps for download in the past three days or so.  I have changed nothing on my router settings or moved the router's location.  I have a Motorola sbg6580 cable modem.  My data usage is only 75GB of my allotted 400GB so I have not gone over limits causing a throttling back of my signal.  These speeds remain consistent for all times of day not just during peak times as well.

Any help will be greatly appreciated.

Thanks,

bj218