Try two things. First, if you have a different key try that. Also, at any parts store you can get a lubericant for tumblers. Get that and spray that in the tumbler to see if it helps. If it doesn't, you will need to remove the tumbler to see if it is bad or the linkage to the ignition switch.
Let me know if you have other questions.
Joe
Feb 8, 2010 at 9:00 PM