I am gonna put this here but i am gonna throw disclaimers on it. I didnt do anything but find this. I have not tried it because I do not have this problem. I do not know if it works. I am not responsible for anything that happens.
I have been having this problem as well. It seems to be a software issue and was happening frequently and under a wide variety of circumstances-texting people, reading emails, watching movies, etc. After some poking around, I have found a possible solution posted by another user that you may want to try. This involves using the commands in the phone dialer to update the key software.
Steps:
1-Open phone dialer
2-Press the buttons INSIDE the parenthesis (*#*#2663#*#*)
3-Look at 2 different touch screen firmware versions.
4-I am too lazy to rewrite the screen here, but you dont need that info. Just look at the version number. If they match then you have another problem. If they do not match 22/21 etc, then update them one at a time and your problem should be fixed.
5-Remember, everything is a double edged sword. Yeah it seems bad when code is not well done and companies should make a better effort to fix their software, but at least samsung was trying to give us something extra that other phones dont have. The screen capture can be a problem if hackers are able to use it to take captures of contact data, sensative messages/photos/info. But remember, alot of people have been asking for this kind of feature...HA HA HA. Anyways, hopefully this fixes it for you, it did for me.
Original found post:
Screen Capture is annoying me!!!! - Android Forums