I never played with my NIC settings. Personally, if 4.5.8.2 was the only change made in your environment that's otherwise had no issues, I'd be inclined to believe it was related to that upgrade. I'm having some weird login issues that nobody else is having, and the only change on my end was 4.5.8.2.
Hopefully when this happens to one of your cameras, it shows up on Statistics > Cameras (and the NoSignal column). Would make it easy to monitor for.