Couple of things to check:
1) Network options.
Ensure that the networks allow option rfc1323 (no -o rfc1323=1).
Also check for the values of thewall, tcp_sendspace, tcp_recvspace & tcp_mssdflt, you may need to change them.
What is the base hardware of the system? Note, MCA boxes have a limit of 262144 for thewall .
What is the oslevel?
How much memory do you have?
2) Try running a traceroute between source/target systems to show if the mtu is being renegotiated.