One thought.
Also just for clarification, are you getting a CPU LED fault on the 06's? Just one or all?
If you are, then my guess (and it is only a guess) is that you just might have some noise getting into the communications either in the serial port or the ECOM and since you are doing a data write, its probably stomping on someplace in memory that is faulting out the CPU. Take another look at the shielding on the serial port and the network cables. If you are using Shielded Ethernet cable assemblies, look at the grounding of the switches. The ground on the ECOM is isolated (I think). However, if all the switches are grounded then the cables between switches would be grounded at both ends. It could also be a faulty switch. Just because its new, it doesn't necessarily mean that its beyond suspect. Also, what protocol are you using on the Serial port? I would disable all the ones that are not used.