Ask Slashdot: How Can I Prove My ISP Slows Certain Traffic?

Long-time Slashdot reader GerryGilmore is “a basically pretty knowledgeable Linux guy totally comfortable with the command line.” But unfortunately, he lives in north Georgia, “where we have a monopoly ISP provider…whose service overall could charitably be described as iffy.” Sometimes, I have noticed that certain services like Netflix and/or HBONow will be ridiculously slow, but — when I run an internet speed test from my Linux laptop — the basic throughput is what it’s supposed to be for my DSL service. That is, about 3Mbps due to my distance from the nearest CO. Other basic web browsing seems to be fine… I don’t know enough about network tracing to be able to identify where/why such severe slowdowns in certain circumstances are occurring.
Slashdot reader darkharlequin has also noticed a speed decrease on Comcast “that magickally resolves when I run internet speed tests.” But if the original submitter’s ultimate goal is delivering evidence to his local legislators so they can pressure on his ISP — what evidence is there? Leave your best answers in the comments. How can he prove his ISP is slowing certain traffic?