TomatoUSB

I got the Router Asus RT-N66U and i got some strange problems:

My Internet Speed is 35MBit/4Mbit:
My PC is connected through LAN-Port:

When i speedtest(dot)net over LAN (with Cat5e) and WLAN off i got full speed 35Mbit/4Mbit
but when i speedtest(dot)net over LAN (with Cat5e) with WLAN on, the rate is dropping to 4-5Mbit/4Mbit on both (LAN and WLAN)

When i switch of WLAN rate is up again @ 35/4….

I've found this problem in Stock Firmware "RT-N66U_3.0.0.4_260" but also in "tomato-K26USB-1.28.RT-N5x-MIPSR2-102-AIO-64K"

Does anyone know, why speed (on LAN and WLAN) is droping to a bad 4-5MBit/4MBit when i turn on WLAN on my router?

Yours MaxiMal1st