Sorry to mislead you I can't find a way to change this in current Solaris or AIX. We really need Solaris to use 1k (not 8k) block size to talk to AIX. Others may know better.

The problem is often caused by ftp incorrectly negotiating (or defaulting) comms parameters to values only appplicable to that manufacturer on a fast LAN.

LAN speed auto-negotiation can cause this type of fault too.

It can be caused by routers incorrectly negotiating large packet sizes when not all routers on the route can deal with them.

The list is endless but usually reducing the ftp block size fixes the problem.