Ignition Key Problem!
Sounds like it may just be the ignition lock tumbler that is at fault. rather than the lock or the ignition switch itself, but you will know for sure once you remove the tumbler. Because this situation can be a much larger problem if the tumbler completely fails and no longer turns at all, you should take the oppurtunity to remove the tumbler while you can still get the key to turn.
Use the search capability above and right and you should find more than a couple threads with instructions. If you'd like get me your e-mail address and I can send you the procedure as a PDF file from my CD you'd just need Acrobat Reader to veiw it! Good Luck!