Fact is, I used to work for my ISP for 2 years, so I knew EXACTLY what they see, know, can or cannot do
Victory was inevitable.
But the best thing to do is to tell them you tried everything and that you're sure the problem is on their side. That usually gets them checking.
Best to say your local network works flawlessly, you turned off all your antivirus/firewall, you checked for viruses, you closed down any torrent software/other software that might hurt bandwith.