His diagnosis was that the network adapter built into the laptop is failing. I concur, but what can I do to try to assure myself that is the case. I've run the Network Troubleshooter which detected no problems. I've also examined the device under Device Manager. It, too, shows that the adapter is performing without fault.
Is there anything else that might be causing the 5 GHz band to disappear and the 2.4 band to deliver such atrocious speeds? I think we exonerated the cable modem and the router using his laptop. My iPhone and iPad both access the networks without apparent problems, although I haven't attempted to measure speed. All 3 TV sets operate over the wireless network with 2 of them at 5GHz and 1 at 2.4. Are there any other diagnostic steps I should try?
