I do have the exact same problem, but with a newly created world.
Sysspecs are:
ESXi virtualized environment. Physical specs: 8x Cores (2x Xeon E5602), 24Gb RAM.
Virtual server for MC: 4x cores, 12Gb RAM, Ubuntu Linux x64 (Java heap size 8G max, 4G min), running MC from ramdisk. Pack is...