I've run into an intermittent problem with my 2019 refurb iMac running Mojave 10.14.6. I connect to the net via ethernet through a Netgear AC2300 router. Every once in a while I am unable to access websites using Safari or Firefox. Blank page and eventual timeout notification. At the same time, I can still receive email through Mail or Postbox, and applications that access the net directly such as Banktivity and StockMarketEye continue to work. The problem occurs whether I have WiFi on or off. Sometimes restarting works, and resetting parameter RAM also seems to do the trick. Hard to tell because it's so random. Oddly enough the last time this happened, my HDHomerun which is also wired to the router stopped working.
I have a 2015 refurb iMac running as well on the same network/router by ethernet – most current system, etc. and it hasn't had the problem.
I've tried a factory reset on the router as well as just turning it on and off, and that doesn't seem to have an effect.
Any ideas/suggestions would be appreciated!