Users behind Juniper firewall can access network for about 1 1/2 to 2 hours, then all applications "freeze" up, e-mail, telnet sessions, etc... To resolve this, the Juniper SSG5 needs to be re-booted. I am not a network admin or tech, just an IT person trying to help out a peer by asking this.

Yeah could be multiple things.. I would first suggest taking a look at event viewer, and any gateway router logs that. Speciafically look for entries around the time it happens will give you furthur insight to what is causeing issue