Annoying problem with wireless router

[Blackberry Bun]

Sometimes when my brother is using my wireless router for browsing or playing online game he just suddenly disconnect and cannot connect to my network anymore with message "Windows is unable to connect to the selected network". Turning off the router and turning it back on will enable him to connect again, but it's annoying whenever that happens. Not to mention that lately it happens twice a day. Anybody has any idea why it happens and what can be done to prevent it?