I must have had ten modem-routers-switches fail in the last year. They seem to carry on working (usually) as router-switches so I generally have network but they cease to access the internet and the web interface ceases to be addressable (usually on the standard 192.168.0.1).

My system is pretty ordinary. ADSL to modem, the two downstairs computers on the built-in switch, ethernet link to a separate switch for the network storage and the two network printers, and ethernet link to a high-power "ethernet over power" gadget (Solwise) that connects to two more such gadgets upstairs, one for the computer in my library and one for my lodger who has another router in his room to connect his computer, his network storage, his X-box and his play-station.

All such things are on spike/surge protectors. My printers and monitors are not.

The failure pattern is the same every time. I put on a new modem-router-switch and once I have persuaded it to talk to the other switch everything is fine for a while. Then I start getting the modem-router-switch failing to give internet access (or even sometimes failing to give internet access to some computers) but working again if I power down and give it a minute or so presumably to cool down. Then I re-power it and away we go. Eventually the internet disappears completely, usually leaving (as I said) a network with no internet access and no access to the router's web interface.   

The latest to do it is a D-link - I'm currently running on my spare Netgear, and I have one more spare Netgear so it's nearly time to go back to E-bay and buy some more spares.

I'm open to suggestions what could be causing it.