Two routers have gone bad, constantly dropping wired connection to PC

Six months ago, a customer had a problem where the router kept dropping the wired connection to the PC about every 30 secs.   I did lots of troubleshooting, e.g. of PC firewall, router firmware, checking the cable modem was ok. The only bad part seemed to be the router. I confirmed this by swapping in a test router, which fixed the problem.  So I bought them a new Netgear WPN824v2 wireless router. It has worked fine for the last six months.

Now the problem has returned -- on the new router (the WPN824v2).  At first, the router will be working fine and the lights will be blinking on the top.  Then the light pattern will change and go static (stop blinking).  Then the PC sees no connection to router at all.  Then after 30 secs or a minute, to goes back to normal and the pattern repeats.

Of course I re-set the router.  The first time it worked fine for 8 mins, then problem returned. I re-set it again. This time it went bad again after about 4 mins.

As I did last time, I swapped in my test router (a linksys) and the problem is gone. So I'm assuming it's definitely the router -- again.

Final note -- I noticed that when I had the bad router(s) in place, under Network Connections, in addition to the "Lan or High Speed" categorie (which contains "Lan Connection"), there was an additional category called "Internet Gateway", which contained "Internet Connection". That's not there now, with the linksys in place.

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(1) Is there something besides a bad router could account for this behavior?

(2) What could account for both routers going bad in the same way.  (Bad wiring? Bad surge protector? Some kind of nearby electromagnetic field or signal that ruins the routers?)