I had the same problem when i turboed my IS... it's running fine, but when u stop at the traffic lights and go again it cuts off.
I put this down to 2 reasons.. the fuel pump and the way my aftermarket ECU works. At below 5% throttle, the standard ECU is in charge, after that the aftermarket ECU takes care of the fuel. Before i changed to aftermarket intank fuel pump the car kept on stalling. Maybe it was becoz the way the fuel lines were relayed and the pressure wasnt strong enough.
The problem went away after i spent some money on an intank fuel pump.