I hope auto iso isnt like my d300s. You set it to cap at 1600 and you go outside on a glorious day at 11:59am and it always selects iso1600!!! completely useless.
Fortunatly ISO is a little button on the top so easy to adjust manually.
You definately are doing something wrong - or the camera is broken - it works a treat! My guess is you have the ISO set to 1600 - it should be set to the lowest ISO you want the camera to use.