Desktop Computer & Slower Speed with Router

[rodrife]

Hello,
Router: R7900-P Tri-Band
Computer: Desktop - Win8.1
Internet Speed: 400mbps
Connection: Ethernet Cable Cat5E

I recently had my cable company to my home because I noticed my internet speed was at about 225 when I should be getting 400mbps. The tech checked the connection in and outside the residence and no problems were found. The tech then made a direct modem/computer ethernet connection and the speed registered 450mbps. After that, the tech connected a new cable company wifi router and the speed, again, came up at 450mbps to his mobile phone. So it boils down to my Nighthawk R7900-P is the problem. I have tried turning unit off and on, unplugging it and resetting, none of the options have worked.
How can the router be fixed to allow 100% of the bandwith to come through, losing 175 to 200mpbs is substantial!
Thank you.

[plemans]

What modem is it connected to? 

Waht firmware is on the router? 

Make sure access control, qos, parental controls, and traffic monitoring are all disabled.

I'd start with a factory reset and clean install. Push and hold reset for 7-15 seconds while the router is powered on. 

Then do a clean install. (not from a backup) During setup, again, make sure those services don't get enabled.