When bored at work, I used to read a lot of snopes.com; now I read LW instead :-)

But if you read enough of it you'll be able to spot the usual patterns. Also in some of the analyses snopes will put forth theories on why the stories stick around so well (usually by how well they play to our biases).

Their research method seems to be similar to what you did - try to find the root story: a direct, local, at-the-time account of the supposed event. They often seem to run into the same circle as you - everyone has second-hand accounts but noone has the original story.