Iray-Only bottom couple of line pixels are rendered

Hi,

It may sound strange the issue i'm facing with. Many times, while rendering high loaded scenes, the result comes like the following images. I see only the bottom couple of line pixels are rendered when this happens. Also i have found a solution for this on my own. After i initiliaze render, i manually slide down to the bottom pixels in render window and wait there till the iterations start. Then, render starts correctly. Yeah sound weird. I wonder what may this problem be related to ?