OK, so it definitely looks like it was the TV and nothing to do with ClearOS after all.

After going through a factory reset on the TV, it is connecting to the network properly and I'm getting the full gigabit performance from my server. I have no idea what it was doing to kill the network and I guuess I will never know.

I now wonder whether the problem I sa in March was the same thing, or something else.....

Thanks again for your help.