http://forum1.netgear.com/showthread.php?t=59671
This is the information I was looking for. Then I can use my LAN better.
いくつかのモデム・ルーターでは、暗号（セキュリティ）に “WPA-PSK+WPA2-PSK”を指定すると、不安定になるということでした。そのため、PSPの接続はあきらめて、 “WPA2-PSK”にしました。
Not sure if this helps, but I came across this same issue, and in trawling through the settings I spotted that under wifi settings, it had a radio option checked for "WPA-PSK+WPA2-PSK", which is support for two security types. In my experience with other networks, this dual encrpytion option isn’t particularly stable, so I switched it to just the "WPA-PSK (Wi-Fi Protected Access Pre-Shared Key)".