And never found the routers to be the cause. Check one thing though. Be sure the router firmware is up to date.
I have connect 2 routers to extend my network to an exterior building. I have done the usual steps. Reassigned the IP address of the second router to be 1.2 and the first to be 1.1. I have disabled dhcp on the 2nd router. Assigned different wifi channels at opposite ends of the channels for the 2 routers so they don't interfere and then put the Ethernet cable going from lan1 in the first router to lan1 in the 2nd. But my internet now drops constantly on both routers for split seconds.