We have a client with the same issue.
Dell servers with intel nic's with latest 5.5 with patches and Dell drivers; also confirmed firmware on NIC's are up to date.
All connected to an 8212 Core.
A few other tests.. that could be tried
ping from the same subnet as vMotion machine and not across IVR (inter vlan router) if you ping tests are from different subnets.
to see if RARP is taking place quicker on switch at L2 and then see if L3 changes are taking longer.(across vlans)
Did you update the Core firmware and see any changes?
