I had a Belkin Wireless G Plus MIMO Router that started to misbehave (at least, the misbehavior became more common) after I switched to High Valley Internet. Since I wasn't sure if it was a problem on their end or with my router, I borrowed another Belkin wireless router from my daughter. (She recently switched providers, and they included a router in the CPE that provides her service.) This was a much newer router (firmware dated 2011 instead of 2002, if that tells you anything), and it worked perfectly.
I also ordered a brand new Cisco Linksys E1200 router, because my daughter will likely need that router again. It arrived yesterday. I installed it. It did not work, using either the autoconfiguration CD, or when I set everything by hand. So I called up High Frontier's network guy--and he could see the ARP table filling in from their side, so there was some communication going on--but not both directions. Weirdly enough, when we set a static IP address for the WAN side, even using the web interface to the router, I could not ping the WAN side's gateway. Weird!
I put my daughter's Belkin router back in--works perfectly. Something's wrong with the Cisco Linksys router, right? I replaced the Belkin with the new router again, so that I could call up Cisco customer service and politely request a replacement. But this time, when I was installing it, I noticed that the instructions made rather a strong point of using the included Ethernet cable for the WAN to CPE connection. Why? Sure enough: problem solved. It appears that this might be a crossover cable--which means the Cisco Linksys E1200 router has different requirements from many other routers out there.
The guy at High Frontier thought this was very odd, because many routers have the WAN port setup to autodetect and switch, depending on whether the cable is crossover or straight-through. Even more odd: there used to be a convention that crossover Ethernet cables were a distinctive color, often red. This is the same color as all the other Ethernet cables that I can find.
Fascinating, and a warning to other router buyers: if the instructions seem unusually insistent on using the supplied cable...use the supplied cable.