[Sephiroth]

Is an attenuator fitted?

As has been pointed out, 51 is the upper upstream power limit and the system will continue to work until/unless the VM can't read what is sent at 51. In that case, after a number of retries, the modem will soft reset. This would show in the logs as a succession of T3 error followed by a T4 event, which is the reset. I'm assuming that the firmware still reports this stuff.

Your short outages are events caused by downstream issues. If the Post-RS error rate is rising from hour to hour (or indeed minute to minute), then you have data corruption coming into the modem. As this is sporadic, it's unlikely to be the modem itself. It could be anywhere (like a tired laser at the optical node in the street). The key to finding out more is to ask your neighbours if it's happening to them. If so, then it's an area fault; if not then it may be anything from the local cabinet tap point to the drop point at your home.

So thr ate at which Post-RS errors are occurring is the next diagnostic needed.