TCP socket sends garbage but with correct Length

Hi All,
I can send UDP packets (DNS), however when sending TCP (http) it sends garbage of the CORRECT LENGTH in some cases and in some it works fine.

Data to W5500 on SPI is correct every time.
Packets checked with wireshark are sometimes ok sometimes not. Length of the packet is correct every time.
After TCP connection is ESTABLISTED the Sn_TX_WR is random somewhere in whole 0xffff range. This is strange as there were only few automatically generated packets and nothing from my side.

I found out that when I use :
1K fifo it fails in about ~50% of cases
2K fifo it fails in about ~75-80% of cases
4K fifo it fails in about ~90% of cases

I experimented with write pointer, tried different sockets, and many other things but found nothing else.
Thank you for the help. I really need it as this should already be with the customer :frowning:

Check against W5100 datasheet chapter You must be doing something wrong there.

After about 40 hours i fixed the issue. It was stupid mistake/typo in my SPI write function. I accidentally shifted address 16 instead of 8 and thus dropped part of the address. I was than writing correct data to the different address.