Just how I'd approach it but I'd plug my machine (wired) into the closest egress point possible (cable modem, DSL modem router, whatever), open up the Network p

Just how I'd approach it but I'd plug my machine (wired) into the closest egress point possible (cable modem, DSL modem router, whatever), open up the Network pane, run a reliable ping, load up the modem/router's status page, then see exactly what happens at that time. This would let me pick up on disconnects, DHCP lease issues, and a variety of other things in one hit. And if it wasn't any of those, would allow me to explore some other possibilities such as the problem being at the ISP's end.