That's the sign of a worn ignition tumbler.
http://www.peachparts.com/shopforum/showthread.php?p=1065596#post1065596
It's a real simple procedure of turning the key to the "II" position, inserting a paper clip and removing the collar. The first time took me about 15 min. after that you can probably do it in about 5min. The links I pasted will give you more information.
BTW, If you are using brass keys, get rid of them. They are OK for an emergency set but not for everyday use.