Activating WiFi on phone disconnects internet on router

[Montalvo]

I have a home network with two routers (wired and WiFi) connected together. The network works fine...UNTIL I boot my phone (MyTouch 4G) and turn on WiFi. That causes me to lose internet connectivity on the whole network! When I unplug the wired router for 30 seconds, that restores everything, but it's a pain.

Anyone have any ideas about what's causing the problem?