Your TV is in closed captioned mode. You are not in one of the Text modes, but in one of the modes that will fill the screen with a big black box and if this mode were used on that network, you would have text in it. Of course it goes away every time you change stations for a short while. Look for a button on your remote labeled.
I Have A Mitsubishi CS-40505 Television That Comes On Fine But After A Minute, Maybe Less,, A Large Blacked Out Square Covers The Center Of The Screen, If I Change Channels It Goes Away Momentarily,then Its Back How Do I Get It To Go Away So I Can Watch With Out Constantly Flipping Back And Forth?
You are in Closed Caption mode. One that is not usually used. On your remote, if there isn't a button marked "CC" then go into the menu and find the caption mode and turn it off.