Just wanted to share that my issues are completely resolved. I have a fairly complex environment but there were two contributing factors - either EEE or STP (spanning tree) enabled on a switch connected to any MA2500C/D. With EEE enabled anywhere, there are symptoms similar to this. With STP enabled, the symptoms are periodic packet loss (but otherwise completely normal operation outside of these hourly periods of 30 second packet loss), but without any loss indicated by the web ui.