I have observed something similar - and still opposite?
I has worked for months - 60+ Mbit throughput without any noticeable problems!
But this weekend, Untangle stopped routing traffic but I could still ping Untangle and access the web interface from LAN but nothing got routed through...
Eventually Untangle didn't give out DHCP responses so I guessed that Untangle was slowly falling apart. After a reboot it seems to work better but one machine still has problem getting through...