I've had an RFX9600 lock-up on much simpler systems and there was no chance of simultaneous transmissions from multiple units. In my experience the lock-ups occurred when no one was using the system.
Which router are you using? As I recall, the Russound CAM units require a DHCP server. Some routers can't seem to manage complex networks and you'll eventually end up with duplicate IP addresses or very slow response from the DHCP server. Fixing as many IP addresses as possible will help, but replacing a defective router is the best solution. In some cases the ISP supplies the defective router. The only real solution here is to place a working router after the ISP's router. True, it will add a very small latency to Internet traffic, but this is better than taking phone calls and making house calls.
In a few cases I've found network printers that can't seem to play nice with the DHCP server and I'll find them using a duplicate IP address. Fixing the printer's IP address is the solution.
Overall, I find that using DHCP is the easiest path -- as long as everything on the network plays nice. I'm impressed that you are using fixed IP addresses for the 9600's. I use DHCP for Pronto's because I can't remember to change the IP address as I download to each 9600. (or more likely, I am interrupted and forget where I am in the sequence)
