The bike shop I work at builds a fair number of Extracycles and a certain amount of chain slap seems to be the accepted norm. Skipping is obviously a problem, but there's not a really good way to eliminate all traces of wobble from a three foot long chain line. I'd say go look at some commercially produced Extracycles for comparison, but judging by the picture you've pieced it together almost exactly the same way as they do it. One of the above comments mentions shortening your chain and that might be a good idea. Making the chain a little bit shorter than you normally would seems to get rid of a lot of excessive chain motion. Good luck.