Internet good, but many web sites won't load.

Same issue on XPX 13 and XPS 15. Have six computers on site, with very strong ethernet and wifi internet. About a week ago, the XPS laptops would no longer load many web pages, going to the usual, "this site can't be reached". Some sites load perfectly, like google, Facebook, Instagram, email, and others. Most others will not load. I've tried both Ethernet and WiFi, moved computers do different access points and locations. No difference. Other computers, connected to same Ethernet and / or WiFi load perfectly at speeds around 400 Mbps. Sometimes, and briefly, they will load and operate normally, but it's a fool's promise, as they soon revert to same behavior. I've tried the command prompt "netsh" and "winsock refreshes" found on various support boards. No change. Rebooted numerous times. Switched cords, WiFi, ports, etc. No change. So strange, as the internet is strong, many sites load, but most will not. I'm a reasonably sophisticated user, but this has me stumped! Any ideas? Thanks!