That did it! I hate when I miss a fundamental elimination like that.
As for the Xs on the 750 row, here is how I got that:
Clue 9 says that Xidat and Quasqueton are 500 years appart. Both have two potential values on the top row. They overlap at 750AD. Thus you could say:
"The culture that was founded in 750AD is either Xidat or Quasqueton"
Then I look for anything that is neither Xidat nor Quasqueton and mark an X on the 750AD line. As well as the Xidat is not Quasqueton box.
Clue 8 has a similar overlapping set at 500AD.
I shoot myself in the foot with these puzzles. I look for the hard ones on purpose. In Firefox, I hold down the CTRL key and click the "New Game" link about 10-12 times. I then hit CTRL-Tab to see if one of those tabs has a game below a 30% solve rate. I close all the other tabs. This particular puzzle has a 20.9% solve rate. So only 1 in 5 players stick with it. I didn't want to be one of the 4.