Off-hand, I don't see any reason your modem would be causing the speed issue as the Nighthawk CM1200 is considered a multi-gig capable cable modem.
Are you testing the speed through a device that is directly connected from your CM1200 modem to the device you're using to perform the test? (I.E an Ethernet cable connected directly from CM1200 to the device, such as your laptop or PC)
Or do you have a separate wireless router connected to the cable modem and are connecting through Wi-Fi?
Just trying to rule out any other possible suspects first.
Hopefully once these few things are clarified we'll have a better idea of what could be causing the problem.
Following up on WMO's points.. Ensure you router is disconnected and powered off, and your computer is directly connected to the modem. reboot the modem, and then test your speed. That's the only accurate way to test your speed.