I don t believe it can be this, but try to disable page sharing for this specific machine. Check http://pubs.vmware.com/vsphere-4-esx-vcenter/index.jsp?topic=/com.vmware.vsphere.resourcemanagement.doc_41/managing_memory_resources/c_sharing_memory_across_virtual_machines.html on how to do it:
"You can also disable sharing for individual virtual machines by setting the sched.mem.pshare.enable option to FALSE (this option defaults to TRUE). See Set Advanced Virtual Machine Attributes."
Check if this works. Also, check if you don't have any limits on the VM CPU/Memory.