My fibre goes really slow for an hour or so then returns to normal

[#1]

I've been having intermittent faults with my fibre broadband for a few weeks now.  Recently getting worse.  I've been running smokeping on my PC to see if it will help identify where the problem is and today lunchtime everything went really slow for over an hour.  Similarly yesterday evening.  I ping the google DNS server at 8.8.8.8 at 30 second intervals, results are plotted on the graph attached.  I similarly ping the router every 30 seconds and no such issue is showing, so I suspect it is not internal wiring.  Anybody any ideas where I can find this problem.

[#2]

Odd, is your PC plugged into the router or wireless?

[#3]

It's plugged in with a cable. The ping to the router doesn't suffer the same effect. Only pinging external sites.