Problem connecting to arduino when ethernet stacked(W5100)

Development Tools IDE 1.x
1

So i have the arduino board, and ethernet shield for quite some time, and they both worked perfectly in the past.
Now i tried using them again for something, but whenever i stack the ethernet shield on top of the arduino, it loses connection, meaning arduino ide cannot see it anymore.
eventually i noticed it only happens, when the ethernet cable is plugged into the shield.
So basically plugging the ethernet cable into the ethernet shield makes arduino ide not see it anymore, removing the ethernet cable and it works perfectly.
What can i do about this?

I tried both an arduino duemilanove, Mega, Mac and windows.
all give same problems.
Its a wiznet w5100 shield by dfrobot, is it faulty?

2

You should not attempt to stack shields with the power applied - each board requires some time to charge up its power-supply decoupling capacitors from 0V, so when you plug it in the supply is temporarily reduced probably resetting both the Arduino and its USB-serial chip...