R6300 Ethernet slow connection

This router has worked great for the past three years. I have a new ISP (Grande Communications) getting 400 Mbps and now all of a sudden the Ethernet ports out of the 6300 router are almost half of what comes out of the modem (Arris DG2470). I've tried all of the router ethernet ports with the same result. I updated the router with the latest firmware three days ago. Tried two new cat 6 cables from the modem to the router and PC. I've tested on multiple computers with the same results. Yet still only get around 180+ Mbps wired out of the router, but am getting 400+ out of the modem. Cat6 from modem to router is six feet. There is only one ethernet cable out of the router plus no active wireless devices devices when I test. The wireless on the Arris modem is disabled so there is no conflict.

I would prefer sticking with a seperate wireless instead of using the wireless on the Arris modem. Plus I use OPEN DNS for family site filterning. (I did a test disabling OPEN DNS with both Google DNS and ISP DNS servers with same results.  I also have two TIVO mini, but they are using the MoCA connection off the modem (not wifi).  The Internet and cable TV come into the house on two seperate feeds.  Grande recently installed fiber in my neighborhood, that's why I left Time Warner who only offered 50Mbps.

Many time the wireless speed is faster than the wired ethernet. Go figure.

I haven't called the cable company yet as they do not support seperate routers.  I suppose I could use their wireless (built into the modem) but prefer having more control over my network.

I'm at a loss as to why this problem occured and need advice on other options to troubleshoot.

Thank you in advance for your comments.