According to IBM it sounds like the connection is timeing out due ot lack of activity.
Resolving the problem
Lower the TCPKEEPALV in CHGTCPA on client system or raise the idle connection timer in the network appliance.
Communications traces (TRCCNN) may be taken concurrently on both the client and server to verify the behavior. Recommended trace output format is .pcap so that network administrators can review using Wireshark.