Hi,
Could be lots of different things obviously.
I'm assuming you don't have speed/duplex hardcoded on the switch ports?
How are the ports configured, are you doing any aggregation on switch? (or just VMWare load balancing by virtual port ID)
Is there a show interface from the switch?
I have seen issues with UCS and drivers for VMWare for certain components.
Worth checking your firmware and ESXi versions.
Are you running Cisco custom image or standard VMWare image?
Cheers,
Tim.