this is the exact same problem i am getting.
Metrofibre fibre at afrihost, from 19h00 till 22h00 same problem, have the 400+ line up and down. neighbor at another provider but still metrofibre not getting the issue
I have the 699 pakage. Speedtest.net gives me a speed of round about 80mps initially but then goes down between 35-50mbps. However netflix and youtube (only 2 i tested) as well as fast.com gets limited to 3mbps.
Still want to test if multiple streams run at the same time i get 3mbps per device...