I updated to 1.8.1 a few days ago, and after about five minutes of Exploring on my horse, the auto save icon pops up in the corner and my horse can't go up blocks and I will be stuck. If I get off my horse and turn the game off/on again, I spawn in my game dead, loosing all my stuff and levels. My horse will be where I left him and continue on horse back for about 5 minutes and then it happens again.

If I get off the horse, walk away from him, turn it off/on again it will be fine for about five minutes, then it happens again.

It is like there is an invisible wall. Mobs get stuck in it and die after the off/on.