deadfoxable

My game keeps crashing and closing without any apparent reason. Nothing specific happens when it crashes, sometimes I am able to play for 3-4 hours without crashes and sometimes only a few minutes. Used MCEdit to prune all chunks outside my base, then deleted all entities within my base. Computer and Java is 64-bit, allocated 6 GB RAM, 16 GB RAM avaible. This is the latest report Modpack crashreport: http://pastebin.com/YwhpbSB7

I am having a problem with my savefile with the modpack Ultra Modded Survival. First there was a problem with my savefile not appearing where you can choose what world to play, so i searched the forums for a solution and found it. The problem now is that every block i place down disappear whenever i quit and enter the world again, but all entities and items in my chest's/inventory/furnace etc. are saved. Does anyone have a solution for this?