Does your RAID card have cache and/or BBU? Whenever I have seen / heard of the symptoms you are explaining it has been the RAID card. Not neccessarily that it is defective, just that its quality is not meant for virtualized workloads. It appears the LSI 2108 is on the VMware compatability list (http://www.vmware.com/resources/compatibility/search.php?deviceCategory=io), but this does not mean that it will perform well when running VMs (I was running a server with the PERC H200, which was also on the list, and it could barely run a single VM - to be fair the H200 has no cache and no BBU).
You could try upgrading the megaraid and LSI firmware. In addition, you could try to upgrade the VIBs on your host (https://my.vmware.com/web/vmware/info/slug/datacenter_cloud_infrastructure/vmware_vsphere/5_1#drivers_tools). Otherwise I would try a higher grade RAID card and/or a server on the HCL.