Did you check to make sure the needle bar is swinging the full range of motion? If it is the least bit gunked up it will cause skipped stitches. Also, switch to cotton 50 wt thread. Some of the old gals don't like poly thread; they were set at the factory for 50wt cotton and to sew cotton. Singer didn't get into the knits until the Touch n Sews of the 70's.