How to capture retransmissions?

[minkie]

One of our hosts has a consistently high level of retransmissions on it's ethernet interface (20%).

I'm spanning a port on our switch so that I can capture the traffic without affecting the performance of the host.

The speed/duplex is OK and there are no errors shown on the switch.

I can capture the traffic using snoop/tcpdump, but is there a simple way of identifying if there is a common source of the retransmissions.

Thanks,
Mike.