Modem drop to very slow connection Motorola SB 6141

I have replaced my older Motorola gateway with a new Motorola SB 6141 modem and a Airport Extreme Router.  THe problem is the modem keeps dropping to a almost unusable speed. It is still connected but very slow.  I have unpluged the modem and let it start back up and it works fine for a while.  I never know when I come back to the computer if the modem  is going to be working or not.