SG-2440 stops working

[flashback]

In December I bought five sg2440 appliances to replace aging Firebox systems. I have deployed four of them in my studios with no problems. They are quiet and quick. This last unit, though is giving me trouble.

After about two hours running the webgui stops responding, and we no longer can access the internet.
SSH doesn't work. Strangely, I can access the device from an external IP (WAN)

My network has three workstations (static ip) a video security sysytem, one wireless device and some small devices for a sales area. At this location I have run a Watchguard Firebox  750e with pfSense (same batch of hardware)  for 2 years with never an issue.

I have checked cables, switches (replaced one) and connectors. I reset the sg2440 to factory defaults twice and reloaded the cfg. and this works, but the problem recurs after about two hours. My configuration is pretty basic… I run RDP, the video, WOL to the workstations.

I finally registered this one box with pfSense support, but I thought I give it a shot here first.