I have had that same problem I use a juki and have found first check all connections second unthread everything . CLEAN CLEAN CLEAN,not just your machine but your rails and wheels on teh stitch regulator that sit on the rails. Eat a good share of chocolate.,sit a spell and then when you have settled down go back look at your setup and make sure you have everything nice and clean then start to rethread top and bottom do a little practice spot .Lots of times I find the wheels sometimes build up some gunk and so do the rails.Hope this helps and hope things work out for you .