but the FEC errors appears to be interesting as every 20 minutes it appears to be in the 40000-50000 range and momentarily drops to around 4500.
I'm not sure I've come across a graph quite like this. Almost metronomic in duration, and patterns to the overlaid "high" and "low" FEC values too...
A quick sample check on the length of those periods:
Yesterday afternoon: "High FEC" for 34 minutes, "Low FEC" for 13, 14, 15 or 18 minutes.
After midnight last night: "High FEC" for 33 minutes, "Low FEC" for 13, 14, 15 or 17 minutes.
Something switched looks to be inducing impulse noise onto the line. Something controlled. Perhaps a water/sewage pump? A heating controller? An air-conditioning controller? A fridge or freezer?
Because the duration is almost identical in the middle of the day compared to overnight, I'm doubting that it is a heating or air-conditioning issue.