I have the Viking 830 and sew daily. I've put decorative stitches in denim and it didn't skip.
You got a lemon. Give your dealer one last shot at replacing your machine; if he/she won't, contact Viking directly. They will not put up with a dealer who can't satisfy customers and gives all Viking machines a bad name. Companies know that word-of-mouth sells products and your problems would prevent others from buying them.
You've probably been very nice to your dealer but it's time to get "down and dirty" and demand satifaction-you've been more than patient. That's too much money and time wasted.
Good luck and let us know what happens!!