Second Screen Bugging on el Capitan

Hi. Ever since the update to el Capitan on my iMac, my second screen has been buggy. It starts when the game loads. The is a constant flicker on the screen that misplaces whatever is displaid. It looks like its lagging but that's only the loading screen. When I get in-game, the second screen freezes and the flickering intensifies. More over, before the update i would occasionally hit my second screen with my cursor. But now, my cursor will stay on the other screen making my cursor disappear from the game. This all has never before been a problem. The only and also fairly simple solution I discovered was just unplugging the screen. I just like to have it there to look at a build or whatever. I hope I'm being clear enough and that there is something that can be done. Kind regards,

FederalDJonos.