I don't have time right now to do the experiments people are talking about, but I wanted to throw out a possible clue.
I've just started getting this hitching this morning, and started wondering what might have changed to cause it. For what it's worth, yesterday I rebooted the computer after a 2-3 weeks, and it installed a Microsoft update. This morning, I opened Chrome (with about 25 tabs open) for the first time since the update. Chrome definitely seems to be a big problem here, although all those tabs were open before the update. Not sure if that helps.
