Skipped stitches and needles breaking while sewing are an indication that the timing is off for your sewing machine. This may sound intimidating, but repairing the timing is actually relatively. Always be sure the power cord has been disconnected from any electrical source before servicing your machine.
Adjusting the timing on an older model sewing machine with a shuttle hook
Disconnect the sewing machine from any power source. Loosen the thumb screw on the left side of the sewing machine body and remove the cover to expose the needle bar. This is the shaft that extends down and that you attach a needle to for sewing. Open the shuttle race and remove the bobbin and bobbin case.
Turn the hand wheel toward you until the needle has reached the lowest point of the stroke. The hook should be at the top of its stroke inside the shuttle race. Loosen the set screw for the needle bar on the upper part of the machine, this is located on the needle bar bushing (the collar that the needle bar slides through when making a stitch). The needle bar should slide easily up and down.
Turning the wheel again, adjust the position of the tip of the needle so that the upper part of the needle eye is just below the bottom edge of the hook inside the shuttle race. Once in position, re-tighten the set screw on the needle bar bushing. Do not allow the needle bar to slip while doing this.
Replace the bobbin and bobbin case and close the shuttle race. Thread and plug in your machine. Using a sizable scrap of cloth, sew a seam to check to see if there are any skips in the stitching. If there are still skips re-adjust the timing again until you can sew without any skipped stitches.
When proper timing has been achieved replace the cover on the needle bar and tighten the thumb screw.