When I first got FTTC and still on BT retail, in the first year of service a problem developed where randomly during the night I would get a load of packet loss that lasted for a short while. It would disappear as fast as it appeared.
I cannot remember all the specific's but what I do remember is this.
It was service affecting hence the term "packet loss", and if I remember right CRC errors did not accumulate on the line, so it was like some kind of invisible problem on the line not registering on the stats, snrm didnt fluctuate etc.
An engineer came, said he did some checks and found nothing wrong but then the problem mysteriously never came back after the engineer visit.