If it's skipping stitches, the timing could be off just a little. Sometimes all it takes is to put a new needle in and make sure it's pushed all the way up, and that the groove is facing toward you. The only reason you would want to replace the hook assembly, is if it's been damaged and has major scars on it. Usually we can polish any small scratches with a very fine sandpaper, about 400 to 600 grit.
Two timing settings to check first, are the needle/hook distance and the loop lift settings. When the point of the hook comes around to the back of the needle, it should cross at the top of the eye of the needle. Also, there should only be a breath of air between the hook and the needle. They should almost touch.