Issues With Benq Xl2420te - It Turns Off On It Own When Watching A Movie

It happens when im watching something covered with very dark background - dark meaning most of the screen is covered by 80% black (or near black colours). It turns off within 3 seconds when it enter these kind of scenes. If you have trouble understanding what i mean above... I open paint and maximize so it covers my entire monitor, fill the entire paint with black colour so about 80% of monitor is covered with black colour, then monitor turns off after 3 seconds or so withour pressing anything. I tried playing with setting but i cant not figure out how to fix it. ITS NOT SCREENSAVOUR Please help thanks! Im ESL.. sorry for my poor english.