If the firewall guys are sure of that then it is likely to be the keep alive settings as mentioned earlier but could you also check the following:
1. Do you have the client read timeout on the Media Servers also set higher than 15 minutes
2. Do you have correct host name resolution through the firewall - can you ping the client from the Master and Media Severs and the Master and Media from the clients (I am thinking you have the client connect / read timeouts set to 15 minutes and that they are not always resolving correctly - so do you use hosts files or rely on DNS)
3. Does the client have multiple network interfaces? if so than again hosts files may help
Not sure of exactly how the settings are done on AIX but on other systems i use these keep alive settings:
# echo 510 > /proc/sys/net/ipv4/tcp_keepalive_time
# echo 3 > /proc/sys/net/ipv4/tcp_keepalive_intvl
# echo 3 > /proc/sys/net/ipv4/tcp_keepalive_probes
Let us know the answer to my questions - hope this helps