To, maybe, narrow down the problem, can you take some of your strips to another machine or to a LQS and try the seams there? If they are O.K. you'll know it's your machine. If they still curve, ask someone else to try. If they STILL curve, try using identical fabric for both pieces........ Well, I hope you get the picture. It does sound like a feed problem. On my first machine I had to keep a little pressure on the bottom fabric to keep even.