Epic working to pinpoint the problem
Reports have been circulating within the Gears of War 3 forums that talk of freezing issues with the Xbox console. Apparently, the freezing seems to happen when players try to load a game mode when in split screen Horde play using two different profiles. When these conditions are met, it seems that is when the bug strikes.
Epic has been asking questions in the forums and trying to gather as much data as possible to troubleshoot and resolve the issue. Right now, Epic suggests you might want to avoid Horde mode in split screen.
Getting the bug swatted is a priority for Epic and they are using the info that they have collected from the community to help formulate a strategy for resolving this matter. In the meantime, know that Epic is working on it; currently, they don’t have an estimate as to when it might be fixed.