Riding minecarts in LAN play seems to break saves?

In the latest snapshot, I've been playing on LAN play with a friend. Things were going pretty well until I started building a long minecart track.

It seems that as I ride a minecart, chunks start to stop being saved? I didn't have any problems before I started riding a minecart, and as soon as I do, whenever I leave the game, it reverts some chunks to an earlier state. It's very strange. Is anyone else getting this problem, or something similar?

I'm using version 12w27a on Windows 7, 32-bit. 4 GB RAM, Single-Core AMD Sempron 2.1 GHz, ATI Radeon 3100