If 51 did not get anything, it will not send an ACK. So it may happen that W5100 sends packets, some of them got lost (probably one), then it sends another packet, and PC sees it is out of order and sends dup ACK. Why 3 seconds… no idea. The best test is to have direct connect to exclude the switch.
Can you please point to exact differences? I think if they are material we must raise them to the WIZnet support.
I recall I had issues when deactivating CS and control signals (RD or WR) at different time. After days, or even weeks of struggle I found out that I must deactivate both at the same time.
