The host that run the vCenter server has 2 two nics connected to Cisco 6500 switch. both adaptors are running "Auto" and is negotiated to max. speed and duplex (full duplex, 1G).
The blade servers are running in HP C7000 with Virtual Connect (FW. 3.70) - Speed and Duplex is managed in VC. (10Gig).
I would like to highlight that this problem started right after upgrading the hosts from ESXi 5.0.1 to 5.1 !!!
We haven't seen it before - the network config. hasn't change doing the upgrade period!