Have you checked the speed/duplex configuration on you switch that the test machines connect to?
You may be seeing problems with a mismatch and that cause input and CRC errors. Just because the Nic shows running at 10MB it will still perform as fast as your physical networking structure.
I would recommend checking the settings on the switches especially if you have it hardcore the speed/duplex on the VM side. Some switches would shut the ports down when there is a mismatch and some would also degrade the performance.
Hope this helps.
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