Twice now I have made a map, spend hours on it, making a house and such, wiring it and make a few machines, I save the world and close minecraft, I come back usually to something like the attached file below, you can clearly see where I had a solar farm and a building, I come back and it's like half the chunk/build gets deleted.
I got the recent modloader, MCforge client 3.1.2.94, and TMI 1.2.5