new router will not stay connected to Arris cable modem & modem resets

I haven't had problems swapping routers until now.  I just bought a D-Link DSR-250 which is an 8 port wired vpn router (I am told the VPN is off by default and it was left off) and am connecting it to the Arris Surfboard 6141.  After doing a few complete power cycles on the modem (30+ minutes unplugged) it will connect with the router and I'll get functional internet for a couple of minutes until the router loses connection with the modem and then the modem physically resets itself.   Thinking it was a problem with the router I also tried buying a new TPLink wired router and the same thing happens.  After a 30+ minute unplug of the modem it'll connect with the router and get internet for a few minutes and then lose connection and the modem resets.  When I go back to try my old routers the modem and internet work fine.  D-Link support says it's most likely an issue with comcast/xfinity.  Any ideas?  Does comcast need to switch something on their end?