Blocks in a specific chunk disappear after exiting game.

Placed blocks within the area marked in red (the area could be bigger, but I've only build in that area) disappear whenever I exit the game and open it again. In 1.4.6 something else happened. Whenever I walked to the edge of the blocks, both grass and bricks, I got stuck and started moving up and down in a buggy way. Jumping a few times would get me unstuck. Also in 1.4.6, this happened.

1.Place grass blocks.
2.Exit
3.Open game, blocks are gone.
4.Place bricks where I had placed grass blocks last time.
5.Bricks transformed into grass blocks as I placed them.

In 1.4.7, blocks still disappear, but the blocks don't transform anymore, and I don't get stuck at the edge either. The only persistent bug is the one of the blocks disappearing, only in that area.

I thought it was some sort of chunk error, but after updating the jar several times, I don't know what could be triggering it.