So your using iSCSI for storage? are the uplinks for iSCSI storage on different physical vSwitchies, nic's from your VM network ? Could you explain more on how you have your vSwitches, Port Groups, and Uplinks configured for VM network, iSCSI, VMkernel, VMotion etc.

Typically you want your iSCSI on a different physical uplink ports from your other traffic then you also have the option of doing Jumbo packets as well which typically helps performance with iSCSI. Or is this 10gE ? are you using vlans at all?