Did you put a new needle in the tinging one? It could have a slight bow in it. The machine could be out of timing, but I'd replace the needle first.
Most, if not all, Singers DSM use a 15X1 needle. They come in all kinds of points and weights which is where the other number comes into play. Sergers use a different needle. I use a size 11 sharp for my piecing and quilting.