World Resetting (but not inventory)?

[gamermanh]

Anyone know why the latest version of Tekkit is suddenly resetting my placed blocks (and the inventories of chests/chest like objects) but not my personal items? It gets laggy, doing a sort of 3 steps forward 2 back thing, then will either crash (or if I leave even when it's not doing this) and come back all block changes are undone. I'm still standing where I was with the same stuff I had, but all my hard work is gone.