Your graphs are perfect.
You can stream without issue.
ISP tests are okay.
The obvious conclusion is hardware issues .
( Or an antivirus program that is badly configured or corrupted, or conflicting with another security program)
Connect another PC or laptop to your router and see what happens.
: Synch : 80/20 : Grade A : Download 75.31.Mb/s : Upload 18.71 Mb/s : ping 9 ms