have a problem with TCP retransmits between a server on a 4507 and clients on 3750 24 port POE.

Previously I had the following setup

Server (fibre gig connected) ->1000-> 4507 (6 port gig card) ->100-> WS-C3550-24 ->100->7204 (NPE200)->100->WS-C2924C-XL->100->PC

This ran fine, the users are uploading big Excel SS's from the server and they could open a SS in 2 mins. Sniffers on the network showed no TCP retransmits

I now have

Server (fibre gig connected) ->1000-> 4507 (6 port gig card) ->1000-> WS-C3750-24P ->100->PC

The users now take 20 mins to open the same SS and the sniffer shows 10 -30 TCP retransmits / second

When I put a sniffer on each end of the fibre between the 4500 and 3700 (mirrored to 100Mb copper ports), it indicates that I am losing packets over the link

I have check for duplex issues and there are none

I cannot see any errored packets or queue drops

I have routed the traffic over different interfaces and fibres and get the same issues

If I connect the PC to a 100mb port on the same 4500 as the server it works fine