Intermittent Nighthawck X6S R8000P

I am having intermittent problem in connecting to various internet & imap servers. When this happens 2 or 3 reconnect attempts will result in connecting. Once connection is made - no problems. Have tried hard reboots to computer & modem(s), factory resets etc. Ethernet connection to router and then to Arris Surfboard SB6190 cable modem on the Xfinity network. Have same problem when I connect 2 other computers to the Nighthawk. All 3 running windows 10 pro and Firefox. Also tried switching cable modems.  When bypassing the Nighthawk and running cable straight to the cable modem have not been able to duplicate the issue as yet but not run tried this for long period as we need the router for wifi.

Have had no problems of this nature until last month or so. Not sure when problem started but I think it may have been about the same time when a Netgear hardware and/or software update was installed. While I usually refrain from hardware updates unless really necessary, I succumbed in this case as never had problems over many years with Netgear. Version now running  - V1.0.4.74_10.1.80  

I would like to try going back to previous version(s) but don’t know how. Unfortunately, just got rid of previous standby router that had been collecting dust.

  I don’t recall making any changes via the routerlogin program. It does not show anything out of order. I can look at the ‘Advanced’ settings but don’t really know what changes if any would help. Due intermittent nature most test programs will not spot problem.

Appreciate your input.