The problem with the needle not coming down in the center is probably a clean and lubricate problem. Push just above the needle on that bar. It should go back and forth easily. If not, everything that moves (or should move) between the needle and the cam stack should be cleaned and oiled until it moves easily. Ditto behind the lever that controls the width of the zigzag.
Sometimes it takes more than one day to achieve "moves easily" as those 400 machines were built to tight tolerances. Almost all oils will dry up and leave a residue over time. Those dried oils are usually the problem with what you describe.
bkay