SAMSUNG LED TV DISPLAY PROBLEM ON START UP  

I have a samsung led tv ua40d5000. its screen quality its excellent, works just fine when consistently running. but it ,sometimes, get this problem that when i turn it on its upper half of screen shows shadowy bar which is darker than the lower half. it goes away when i make the tv straight or a little bent forwards. actually i dont have stand for led and i have supported it against a partial wooden support so that it tilts a little backwards about 3-5 degrees. and when i turn it on it shows that problem which goes away by changing its posture. and it only occurs after a cold start. i wonder if it is due to wrong positioning of the tv or some other problem. HELP PLEASE! regards