The Great Gatsby, the formless grace of our nervous, sporadic games

I'd like to understand the meaning of the phrase "the formless grace of our nervous, sporadic games" in the following excerpt of The chapter 4 of The Great Gatsby, especially the precise significance of the words "formless" and "sporadic".

He was balancing himself on the dashboard of his car with that resourcefulness of movement that is so peculiarly American—that comes, I suppose, with the absence of lifting work or rigid sitting in youth and, even more, with the formless grace of our nervous, sporadic games. This quality was continually breaking through his punctilious manner in the shape of restlessness. He was never quite still; there was always a tapping foot somewhere or the impatient opening and closing of a hand.

Thank you.