Remote telnet sessions locks up with larger amount of output

Connected remotely (through a Sonicwall router) entering commands with a lot of output, such as ls -al or ps -ef, the sessions just locks up. Entering other commands, with less output seem to run fine. If I connect directly from the console I don't have this issue. We have tried lowering the MTU and still have the problem. Not sure if there is a firewall appliance problem or not. We have an AIX box on the same network and never have any problems with it. We are running Red Hat EL 6.4.

Any ides would be greatly appreciated. Thanks